drm/i915: Precompute gamma_mode

drm/i915: Precompute gamma_mode

We shouldn't be computing gamma mode during the commit phase.
Move it to the check phase.

v2: Reword comments a bit (Matt)
    Rebase

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/20190205160848.24662-3-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>