drm/etnaviv: fix missing unlock on error in etnaviv_gpu_submit()

Add the missing unlock before return from function etnaviv_gpu_submit()
in the error handling case.

lst: fixed label name.

Fixes: f3cd1b06 ("drm/etnaviv: (re-)protect fence allocation with
GPU mutex")
CC: stable@vger.kernel.org #4.9+
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>