drm: Delete the vblank timer synchronously at cleanup time

A race condition exists in drm_vblank_cleanup() if the vblank disable
timer callback runs after freeing the memory that its callback function
tries to access. Fix this by deleting the timer synchronously.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>