accel/habanalabs: break is_idle function into per-engine sub-routines

is_idle() was too long, so break it up for readability.
In addition, we can now use the new sub-routines from other places.

Signed-off-by: Koby Elbaz <kelbaz@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>