habanalabs: fix NULL pointer dereference

When polling fences for multi CS, it is possible that fence is
no longer exists (its corresponding CS completed and the fence was
deleted) but we still accessing its parameters, causing NULL pointer
dereference.

Fixed by checking if fence exits before accessing its parameters.

Signed-off-by: Dani Liberman <dliberman@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>