drm/arcgpu: Drop drm_vblank_cleanup

CRTC don't seem to get shut down in a controlled fashion, but no one
bothers to stop interrupts either so this races no matter what. Might
as well remove it. A call to drm_atomic_helper_shutdown would be
pretty sweet somewhere (and maybe getting rid of the load/unload
callbacks while at it).

Cc: Alexey Brodkin <abrodkin@synopsys.com>
Reviewed-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524145212.27837-16-daniel.vetter@ffwll.ch
Link: http://patchwork.freedesktop.org/patch/msgid/20170524145212.27837-14-daniel.vetter@ffwll.ch