drm/i915: intel_unregister_dsm_handler() doesn't need struct_mutex

So don't grab the lock before calling the function.

Reviewed-by: Chris wilson <chris@chris-wilson.co.uk>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>