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

Make sure both crtcs participating in the bigjoiner stuff
are 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/20201117194718.11462-8-manasi.d.navare@intel.com