drm/i915: Get the uapi state from the correct plane when bigjoiner is used

When using bigjoiner userspace is only controlling the "master"
plane, so use its uapi state for the "slave" plane as well.

hw.crtc needs a bit of magic since we don't want to copy that from
the uapi state (as it points to the wrong pipe for the "slave
" plane). Instead we pass the right crtc in explicitly but only
assign it when the uapi state indicates the plane to be logically
enabled (ie. uapi.crtc != NULL).

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/20201117194718.11462-10-manasi.d.navare@intel.com