drm/i915: Declutter color key register stuff

Add a few small helpers to calculate the color key register
values. Cleans up skl_program_plane_arm() a bit.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211201152552.7821-14-ville.syrjala@linux.intel.com


Reviewed-by: José Roberto de Souza <jose.souza@intel.com>