drm/i915: Optimize out redundant dbuf slice updates

if the new dbuf slices are a superset of the old
dbuf slices then we don't have to do anything in
intel_dbuf_post_plane_update(). Restructure the code
to skip such redundant dbuf slice updates. The main
benefit is slightly less confusing logs.

Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240402155016.13733-15-ville.syrjala@linux.intel.com