The graphics card (Intel Iris graphics 6100) sends 16:9 video to the TV. 1-2 cm of its borders are cropped. How can I correct this?
This is an undercan issue best being resolved from from setting on your TV. Look for a menu entry to control output screen size from where you may be able to allow adjusting the screen depending on the HDMI input signal.
The menu may be named differently, depending on your brand. For us a menu entry "Automatically adjust size" was the one needed for fully displaying the desktop. Sadly this setting may not always survive a power cycle of the TV set, and it may also be affected by other HDMI devices attached (such as a BD-player).
Reason was the activated TV picture overscan setting of my TV device. If this setting is enabled, the borders of the pictures received from HDMI are cropped. I disabled overscan and now pictures stemming from my Ubuntu PC fit perfectly.
You can read more about overscan here: http://www.komeil.com/blog/fix-cropped-picture-cut-off-screen-tv-hdmi-dvi