habanalabs: refactor MMU locks code

remove mmu_cache_lock as it protects a section which is already
protected by mmu_lock.

in addition, wrap mmu cache invalidate calls in hl_vm_ctx_fini with
mmu_lock.

Signed-off-by: Ohad Sharabi <osharabi@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>