accel/habanalabs: split cdev creation to separate function

Move the cdev creation code from the main hdev init function to
a separate function. This will make the code more readable once we
add the accel registration code (instead/in addition to legacy
cdev).

Signed-off-by: Oded Gabbay <ogabbay@kernel.org>
Reviewed-by: Tomer Tayar <ttayar@habana.ai>
Reviewed-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>