Removed useless locks to DeviceContainer::m_devices_mutex, as they were all already protected by m_devices_population_mutex.
We have no interest in blocking other threads that were potentially reading devices at the same time so this seems fine.
This simplifies the code, and I've adjusted a few comments which mentioned possible deadlock that should now be totally gone.
The deadlock could have happen if a thread directly called EmulatedController::UpdateReferences(), while another another thread also reached EmulatedController::UpdateReferences() within a call to ControllerInterface::UpdateDevices(), as the mentioned function locked both the DeviceContainer::m_devices_mutex and s_get_state_mutex at the same time.
The deadlock was frequent on game emulation startup on Android, due to the UpdateReferences() call in InputConfig::LoadConfig() and the UI thread triggering calls to ControllerInterface::UpdateDevices().
It could also have happened on Desktop if a user pressed "Refresh Devices" manually in the UI while the input config was loading.
Also brought some UpdateReferences() comments and thread safety fixes from https://github.com/dolphin-emu/dolphin/pull/9489
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