drm/i915: reject invalid formats for FBC

This commit is essentially a rewrite of "drm/i915: Check pixel format
for fbc" from Ville Syrjälä. The idea is the same, but the code is
different due to all the changes that happened since his original
patch. So any bugs are due to my bad rewrite.

v2:
  - Drop the alpha formats (Ville).
v3:
  - Drop the stale comment (Ville).

Testcases: igt/kms_frontbuffer_tracking/*fbc*-${format_name}-draw-*
Credits-to: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>