habanalabs: Expose devices after initialization is done

The char devices are currently exposed to user before the device and
driver initialization are done.
This patch moves the cdev and device adding to the system to the end of
the initialization sequence, while keeping the creation of the
structures at the beginning to allow the usage of dev_*().

Signed-off-by: Tomer Tayar <ttayar@habana.ai>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>