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 <[email protected]>
Signed-off-by: Chris Wilson <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Liviu Dudau <[email protected]>
Cc: Daniel Vetter <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]