habanalabs: invalidate MMU cache only once

Reduce context close time by performing MMU cache invalidation once at the
end of the unmap loop rather in each iteration, in order to avoid hard
reset with open contexts.
Reset with open contexts can potentially lead to a kernel crash as the
generic pool of the MMU hops is destroyed while it is not empty because
some unmap operations are not done.
The commit affect mainly when running on simulator.

Signed-off-by: Omer Shpigelman <oshpigelman@habana.ai>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>