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

Fixes: 187d2929 ("drm/panfrost: Add support for GPU heap allocations")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190814044814.102294-1-weiyongjun1@huawei.com