drm/i915: Pull GAMMA_MODE write out from haswell_load_luts()

drm/i915: Pull GAMMA_MODE write out from haswell_load_luts()

For bdw+ let's move the GAMMA_MODE write for the legacy LUT
mode into the .load_luts() funciton directly, rather than
relying on haswell_load_luts(). We'll be getting rid of
haswell_load_luts() entirely soon, and it's anyway cleaner
to have the GAMMA_MODE write in a single place.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190205160848.24662-5-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>