drm/i915/fbc: drop uncore locking around i8xx/i965 fbc nuke

The locking should not be needed after commits de5bd083
("drm/i915/fbc: Skip nuke when flip is pending") and 7cfd1a18
("drm/i915: Remove remaining locks from i9xx plane udpates").

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221205122918.3092092-1-jani.nikula@intel.com