drm/etnaviv: Fix a leak in submit_pin_objects()

If the mapping address is wrong then we have to release the reference to
it before returning -EINVAL.

Fixes: 088880dd ("drm/etnaviv: implement softpin")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>