drm/i915: s/dev_priv/i915/ in intel_color.c

Switch intel_color.c over to the modern 'i915' variable
naming scehme. The only exceptions are the i9xx LUT access
functions which still need the magic 'dev_priv' for the
register macros.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221026113906.10551-4-ville.syrjala@linux.intel.comReviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>