drm/shmob: Drop drm_vblank_cleanup

It doesn't do anything in the driver load error paths that the drm
core doesn't also do (cleanup is done in drm_dev_fini).

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170621082850.13224-10-daniel.vetter@ffwll.ch