drm/i915: Add debugs for the C8 vs. legacy LUT case

Leave a hint in dmesg when we reject a configuration attempting
to use C8 planes without the legacy LUT loaded.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190513133904.20374-3-ville.syrjala@linux.intel.com


Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>