drm/i915: Turn off the CUS when turning off a HDR plane

We're currently leaving the CUS enabled if we disable the
master plane directly after scanning out NV12.

Could perhaps cause the selected slave plane to misbehave
if we try to use it for scanning out something non-NV12?

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190315195445.26527-2-ville.syrjala@linux.intel.com


Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110032