drm/i915: Constify the state arguments to the color management stuff

drm/i915: Constify the state arguments to the color management stuff

Pass the crtc state etc. as const to the color management commit
functions. And while at it polish some of the local variables.

v2: 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/20190205160848.24662-4-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>