drm/i915: Fix overlay colorkey for 30bpp and 8bpp

As with the video sprites the colorkey is always specified
as 8bpc. For 10bpc primary plane formats we just ignore the
two lsbs of each component. For C8 we'll replicate the same
key to each chanel, which is what the hardware wants.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191028113036.27553-3-ville.syrjala@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>