habanalabs: add validity check for signal cs

In preparation for a new feature that allows the user to reserve
signals ahead of submissions, we need to change a current assumption
in the code.

Currently, the driver uses 2 SOBs to support signal CS. When the first
SOB reaches max value, the driver switches to the other one and assumes
that when it will need to switch back to the first one, all of the
signals have already been handled.

This assumption won't hold when the new feature will be added, because
using signal reservation, the driver can reach the max SOB value very
fast.

The change is to add a validity check when submitting a signal CS, to
make sure the previous SOB is available (all the signals attached to
it indeed finished).

Signed-off-by: farah kassabri <fkassabri@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>