drm/etnaviv: return GPU fence through the submit structure

The next patch will need the complete dma_fence, instead of just the seqno,
to create the sync_file in etnaviv_ioctl_gem_submit, in case an
out_fence_fd is requested.

The submit needs to hold a reference to the dma_fence, to avoid raceing
with the GPU completing the fence.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Philipp Zabel <p.zabel@pengutronix.de>
---
New patch in v3.