habanalabs: add cdev index data member

Instead of recalculating the cdev index, store it in a dedicated data
member. This data member is intended to be passed to other drivers using
the auxiliary bus infra and hence this new data member is necessary in
case that the calculation is changed in the future.

Signed-off-by: Omer Shpigelman <oshpigelman@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>