habanalabs: fix possible memory leak in MMU DR fini

This patch fixes what seems to be copy paste error.

We will have a memory leak if the host-resident shadow is NULL (which
will likely happen as the DR and HR are not dependent).

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