drm: Remove "protection" around drm_vma_offset_manager_destroy()

Using a spinlock to serialize the destroy function, within the destroy
function itself does not prevent the buggy driver from shooting
themselves in the foot - either way they still have a use-after-free
issue.
Reported-by: NJia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Liviu Dudau <Liviu.Dudau@arm.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180903093155.3825-1-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_vma_manager.c

@@ -103,10 +103,7 @@ EXPORT_SYMBOL(drm_vma_offset_manager_init);
  */
 void drm_vma_offset_manager_destroy(struct drm_vma_offset_manager *mgr)
 {
-	/* take the lock to protect against buggy drivers */
-	write_lock(&mgr->vm_lock);
 	drm_mm_takedown(&mgr->vm_addr_space_mm);
-	write_unlock(&mgr->vm_lock);
 }
 EXPORT_SYMBOL(drm_vma_offset_manager_destroy);