habanalabs: MMU invalidation h/w is per device

The code used the mmu mutex to protect access to the context's page
tables and invalidation of the MMU cache. Because pgt are per
context, the mmu mutex was a member of the context object.

The problem is that the device has a single MMU invalidation h/w
(per MMU). Therefore, the mmu mutex should not be a property of the
context but a property of the device.

Signed-off-by: Oded Gabbay <ogabbay@kernel.org>