drm/i915/fbc: Fix the plane end Y offset check

drm/i915/fbc: Fix the plane end Y offset check

We lost the required >>16 when I refactored the FBC plane state
checks. Bring it back so the check does what it's supposed to.

Cc: Mika Kahola <mika.kahola@intel.com>
Fixes: 6a58970db50f ("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/20220210103107.24492-1-ville.syrjala@linux.intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
(cherry picked from commit f7bc440bc79ae5dcf648b90209910ea8dba6ef0c)
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>