drm/i915: Store frontbuffer_bits in the plane

Avoid some 'switch (plane->type)' by storing the fronbuffer_bits in
intel_plane.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: use singular frontbuffer_bits in intel_plane since a plan can
only ever have one bit. Discussed with Ville on irc.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>