drm/i915: Reset glk degamma index after programming/readout

Just for some extra consistency let's reset the glk degamma LUT
index back to 0 after we're dong trawling the LUT.

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/20200925131656.10022-6-ville.syrjala@linux.intel.com