ASoC: SOF: ipc4-pcm: Serialize the PCM free with the pipeline triggers

sof_ipc4_pcm_free() frees the array of the pipelines to be triggered
without any serialization against sof_ipc4_trigger_pipelines(), which
walks the very same array. The trigger took the pipeline_state_mutex
only after it fetched the first pipeline from the list, so a PCM free
running in parallel - on unbind or topology removal - can free the array
from under it.

Take the pipeline_state_mutex in sof_ipc4_pcm_free() and move the guard
in sof_ipc4_trigger_pipelines() before the first access to the pipeline
list. Clear the count of the freed list as well, so that a trigger which
was waiting for the mutex sees an empty list and returns.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Liam Girdwood <liam.r.girdwood@intel.com>
Link: https://patch.msgid.link/20261007084955.1256-12-peter.ujfalusi@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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