drm/docs: Record TODO about plane clipping

It's such a mess that it's become a FAQ :(

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Russell King - ARM Linux <linux@armlinux.org.uk>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170226193851.3245-1-daniel.vetter@ffwll.ch