habanalabs: fix double unlock on error in map_device_va()

If hl_mmu_prefetch_cache_range() fails then this code calls
mutex_unlock(&ctx->mmu_lock) when it's no longer holding the mutex.

Fixes: 9e495e24

 ("habanalabs: do MMU prefetch as deferred work")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>