drm/vc4: Correct address offset for planes with src_[xy] offsets

11cf37e7 switched to using drm_fb_dma_get_gem_addr instead of
drm_fb_dma_get_gem_obj and adding fb->offset[].

However the tiled formats need to compute the offset in a more
involved manner than drm_fb_dma_get_gem_addr applies, and we
were ending up with the offset for src_[xy] being applied twice.

Switch back to using drm_fb_dma_get_gem_obj and fully computing
the offsets ourselves.

Fixes: 11cf37e7 ("drm/vc4: Move the buffer offset out of the vc4_plane_state")
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>