ASoC: mediatek: mt8186: Remove unused mutex.

The mutex mutex_request_dram has no user.

Remove mutex_request_dram.

Link: https://lore.kernel.org/r/20230803083908.9DxbPvOK@linutronix.de


Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>