drm/i915: Clean up integer types in color code

A variable called 'i' having an unsigned type is just looking for
trouble, and using a sized type generally makes no sense either.
Change all of them to just plain old int. And do the same for some
'lut_size' variables which generally provide the loop end codition
for 'i'.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200303173313.28117-7-ville.syrjala@linux.intel.com
Reviewed-by: Swati Sharma <swati2.sharma@intel.com>