drm: etnaviv: clean up submit_bo()

As we now store the etnaviv_vram_mapping, we no longer need to store
the iova itself: we can get this directly from the mapping structure.
Arrange for submit_bo() to return a pointer to etnaviv_gem_submit_bo,
and directly access mapping->iova when applying relocations.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>