drm/i915: Extract glk_color_check()

Unlike the earlier platforms GLK has dedicated degamma and gamma
LUTs. And quite curiously the degamma LUT is actually controlled
via the PLANE_COLOR_CTL CSC enable bit. Hence we must compute
gamma_enable and csc_enable differently to pre-GLK platforms.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190327155045.28446-7-ville.syrjala@linux.intel.com