drm/i915: Add a couple WARN()'s to catch missing locks

Add !mutex_is_locked() checks to intel_pin_and_fence_fb_obj() and
intel_unpin_fb_obj() to help catch failures to grab struct_mutex when
operating on fb objects.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>