drm/i915: Fix legacy gamma mode for ICL

drm/i915: Fix legacy gamma mode for ICL

We must remember to actually enable the post CSC gamma if
we expect the legacy LUT to work. Seems to fix NV12 crc
tests on the SDR planes. Curiously we apparently managed to
get 100% match for the HDR planes even without chopping
off the low bits.

Cc: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190315195445.26527-1-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>