habanalabs: make last_mask an MMU property

Currently LAST_MASK is a global, but really it is an MMU implementation
specific. We need this change for future ASICs.

Signed-off-by: Yuri Nudelman <ynudelman@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>