drm/i915: Add planes affected by bigjoiner to the state

Make sure both the bigjoiner "master" and "slave" plane are
in the state whenever either of them is in the state.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201117194718.11462-9-manasi.d.navare@intel.com