drm/i915: Fix GCMAX color register programming

GC MAX register is used to program values from 1.0 to
less than 3.0. A different register was used instead of
the intended one. Fixed the same.

Currently limiting it to 1.0 due to ABI limitations.

v2: Updated the 1.0 programming and aligned as per GLK, based
on Ville's feedback.

Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1553869756-4546-2-git-send-email-uma.shankar@intel.com