drm/sun4i: backend: Remove the MBUS quirks

Now that the MBUS quirks are applied by our global notifier, we can
remove them from our DRM driver.

Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>