drm/i915: Extract skl_plane_{wm,ddb}_reg_val()

Extract helpers to calculate the final wm/ddb register
values for skl+. Will allow me to more cleanly remove the
register write wrappers for these registers.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240510152329.24098-15-ville.syrjala@linux.intel.com


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