drm/sun4i: Rely on dma interconnect for our RAM offset

Now that we can express our DMA topology, rely on those property instead of
hardcoding an offset from the dma_addr_t which wasn't really great.

We still need to add some code to deal with the old DT that would lack that
property, but we move the offset to the DRM device dma_pfn_offset to be
able to rely on just the dma_addr_t associated to the GEM object.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/5df781318e7e05f780a11ed243dcf2b9fe8a08cb.1554108995.git-series.maxime.ripard@bootlin.com