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().

Fixes: 623a6143 ("mailbox: mediatek: Add Mediatek CMDQ driver")
Signed-off-by: Jason-JH.Lin <jason-jh.lin@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>