etnaviv: prevent deadlock in error path

If drmPrimeFDToHandle fails in etna_bo_from_dmabuf, the function must
not return with the table_lock mutex held. Unlock the mutex in the error
path.

Based on ceb70a6b ("freedreno: prevent deadlock in error path").

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>