drm/i915: Turn off pipe CSC when it's not needed

As with pipe gamma we can avoid the potential precision loss from
the pipe csc unit when there is no need to use it. And again
we need the same logic for updating the planes.

v2: Rebase

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190207202146.26423-6-ville.syrjala@linux.intel.com