drm/sun4i: backend: Fix probe failure with multiple backends

Commit e0d07278 ("dma-mapping: introduce DMA range map, supplanting
dma_pfn_offset") introduced a regression in our code since the second
backed to probe will now get -EINVAL back from dma_direct_set_offset and
will prevent the entire DRM device from probing.

Ignore -EINVAL as a temporary measure to get it back working, before
removing that call entirely.

Fixes: e0d07278 ("dma-mapping: introduce DMA range map, supplanting dma_pfn_offset")
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>