drm/i915: Move icl+ nv12 plane register mangling into skl_universal_plane.c

drm/i915: Move icl+ nv12 plane register mangling into skl_universal_plane.c

Try to keep all the low level skl+ universal plane register
details inside skl_universal_plane.c instead of having them
sprinkled all over the place.

v2: Rebase due to intel_display changes

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250212164330.16891-10-ville.syrjala@linux.intel.com