drm/i915: Disallow plane scaling with specific pixel formats

Plane scaling is not supported with specific pixel formats. Disallow
plane scaling when such a format is used. Currently the only such
pixel format we expose is C8, but in case we add more in the future
let's make it easy to deal with them.

v2: Redo due to plane_check() refactoring

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com> #v1
Link: https://patchwork.freedesktop.org/patch/msgid/20181005125817.22576-6-ville.syrjala@linux.intel.com