drm/etnaviv: fix missing unlock on error in etnaviv_iommuv1_context_alloc()

Add the missing unlock before return from function etnaviv_iommuv1_context_alloc()
in the error handling case.

Fixes: 27b67278 ("drm/etnaviv: rework MMU handling")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>