drm/i915: Fix cursor src/dst rectangle with bigjoiner

We can't call drm_plane_state_src() this late for the slave plane since
it would consult the wrong uapi state. We've alreayd done the correct
uapi->hw copy earlier, so let's just preserve the unclipped src/dst
rects using a temp copy across the intel_atomic_plane_check_clipping()
call.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201117194718.11462-14-manasi.d.navare@intel.com