drm/i915: Move single buffered plane register writes to the end

drm/i915: Move single buffered plane register writes to the end

The plane color correction registers are single buffered. So
ideally we would write them at the start of vblank just after the
double buffered plane registers have been latched. Since we have
no convenient way to do that for now let's at least move the
single buffered register writes to happen after the double
buffered registers have been written.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181114210729.16185-3-ville.syrjala@linux.intel.com
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>