drm/etnaviv: unlock on error in etnaviv_gem_get_iova()

We have to drop a lock before returning -ENOMEM here.

Fixes: a8c21a54 ('drm/etnaviv: add initial etnaviv DRM driver')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>