drm/i915: Clean up SKL_BOTTOM_COLOR defines

Use REG_BIT() for SKL_BOTTOM_COLOR.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211112193813.8224-4-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>