The Quick Fix (to force the reverting back to the "xserver-org-video-ati" open source drivers):
We need to make sure that both the "fglrx-core" and "fglrx-updates-core" software packages are removed. Only one of those two will be currently installed. We'll attempt to remove both, just to be safe.
In a Terminal:
sudo apt-get remove fglrx-core
[give your user's password, then enter "y" to proceed]
sudo apt-get remove fglrx-updates-core
Reboot.
Log back in, then go back to "Additional Drivers" (search for that in the Dash). You'll see your video driver choices looking like they used to, and the xserver-org-video-ati choice is selected (not fglrx). Now leave these choices alone!
A longer-winded explanation:
I had the same problem as you, in Ubuntu 15.04 64-bit, but my ATI video card is slightly different:
# lspci -k | grep VGA -A2
00:01.0 VGA compatible controller: Advanced Micro Devices, Inc. \
[AMD/ATI] Richland [Radeon HD 8510G]
Subsystem: Toshiba America Info Systems Device fa16
Kernel driver in use: fglrx_pci
My laptop is a Toshiba L50D-A.
I was stuck myself, with "fglrx_pci" effectively being shown (by lspci) as used (not "radeon", which is what you'll see when the "xserver-org-video-ati" driver is being used in "Software Updates" -> "Additional Drivers"), experiencing very sluggish video performance, and the "Additional Drivers" video driver choices being locked, unchangeably, on "Continue using a manually installed driver".
This suggests the problem is not just with your specific video chipset, but with (some? all?) ATI chipsets in general.
What's shocking to me is that one effectively could not even revert back to the open source driver. That is to say that currently it's a trap, to even attempt to use the provided ATI proprietary driver choices, after a fresh Ubuntu install.
ATI has had a long-standing reputation for having poor proprietary video drivers in Linux. I suppose it was nice of them that they made them at all.
These various Ubuntu forums are rife with ATI problems, and they never really seem to go away. ATI's driver testing employees clearly don't do thorough testing, when it comes to a laptop suspending/waking up (and then switching between users after doing so).
In the future, you'd do well to NOT BUY ATI VIDEO CHIPSETS IN THE FIRST PLACE, if you want to use Ubuntu. This might eventually encourage ATI to spend more on software driver testing for Linux (if they see that they are losing money, because customers are choosing competing products instead).
You'd also do well not to shut-the-laptop-lid-to-suspend, thereby asking for trouble (although this seemed to work OK using the Open Source "xserver-org-video-ati" driver for me. YMMV).
Note: with the proprietary "fglrx_pci" driver, I could get 120 fps in Extreme Tux Racer. With the open source "radeon" driver, I could get 60 fps. That 60 fps is a rather decent framerate for me (in the sense that it sure feels like there IS hardware acceleration), making me not care about taking risks with the ATI proprietary drivers again.
Also Note: My semi-transparent misty Unity Dash once again opens nice and fast with the "radeon" driver as well, compared to a sluggish, no-transparency Dash (experienced during the "stuck, no-acceleration fglrx" problem above).