drm/i915: More cargo-culted locking for intel_update_fbc

Just for consistency, this patch won't fix anything really.

v2: Rebase over all the recent plane enabling shuffling.

Reviewed-by: Akash Goel <akash.goel@intel.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>