drm: Remove "protection" around drm_vma_offset_manager_destroy()

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: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Chris 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: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180903093155.3825-1-chris@chris-wilson.co.uk