habanalabs: share a single ctx-mutex between all MMUs

Multiple locks are usually a source of problems, which in the MMU
case can be avoided since it is relatively rare that both MMU
tables are updated at the same time.

Therefore, use a single shared lock instead of two separate ones.

Signed-off-by: Moti Haimovski <mhaimovski@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>