mailbox: mtk-cmdq: Move devm_mbox_controller_register() after devm_pm_runtime_enable()

In the Linux kernel, the following vulnerability has been resolved:

mailbox: mtk-cmdq: Move devm_mbox_controller_register() after devm_pm_runtime_enable()

When mtk-cmdq unbinds, a WARN_ON message with condition
pm_runtime_get_sync() < 0 occurs.

According to the call tracei below:
cmdq_mbox_shutdown
mbox_free_channel
mbox_controller_unregister
__devm_mbox_controller_unregister
...

The root cause can be deduced to be calling pm_runtime_get_sync() after
calling pm_runtime_disable() as observed below:
1. CMDQ driver uses devm_mbox_controller_register() in cmdq_probe()
to bind the cmdq device to the mbox_controller, so
devm_mbox_controller_unregister() will automatically unregister
the device bound to the mailbox controller when the device-managed
resource is removed. That means devm_mbox_controller_unregister()
and cmdq_mbox_shoutdown() will be called after cmdq_remove().
2. CMDQ driver also uses devm_pm_runtime_enable() in cmdq_probe() after
devm_mbox_controller_register(), so that devm_pm_runtime_disable()
will be called after cmdq_remove(), but before
devm_mbox_controller_unregister().

To fix this problem, cmdq_probe() needs to move
devm_mbox_controller_register() after devm_pm_runtime_enable() to make
devm_pm_runtime_disable() be called after
devm_mbox_controller_unregister().