drm/i915/fbc: Consult hw.crtc instead of uapi.crtc

plane_state->uapi.crtc is not what we want to be looking at.
If bigjoiner is used hw.crtc is what tells us what crtc the plane
is supposedly using.

Not an actual problem on current hardware as the only FBC capable
pipe (A) can't be a bigjoiner slave and thus uapi.crtc==hw.crtc
always here. But when we get more FBC instances this will become
actually important.

Fixes: 2e6c99f8

 ("drm/i915/fbc: Nuke lots of crap from intel_fbc_state_cache")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220413152852.7336-1-ville.syrjala@linux.intel.com
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>