drm/i915: Clean up i9xx_load_luts_internal()

Split i9xx_load_luts_internal() into neat gmch vs. ilk+ chunks.
Avoids at least one branch in the inner loop, and makes life
a bit less confusing.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200303173313.28117-3-ville.syrjala@linux.intel.com
Reviewed-by: Swati Sharma <swati2.sharma@intel.com>