drm/i915: Clean up skl+ plane alpha bits

Convert a few more skl+ plane registers to REG_BIT() & co.
Somehow thse were missed during the earlier cleanup.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230314130255.23273-8-ville.syrjala@linux.intel.com


Reviewed-by: Jouni Högander <jouni.hogander@intel.com>