ALSA: compress: Initialize mutex in snd_compress_new()

Currently the snd_compr.lock mutex isn't initialized in the API
functions although the lock is used many places in other code in
compress offload API.  It's because the object was expected to be
initialized via snd_compress_register(), but this was never used by
ASoC, which is the only user.  Instead, ASoC initializes the mutex by
itself, and this is error-prone.

This patch moves the mutex initialization into the more appropriate
place, snd_compress_new(), for avoiding the missing init.

Reviewed-by: Mark Brown <broonie@kernel.org>
Acked-by: Vinod Koul <vkoul@kernel.org>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Link: https://lore.kernel.org/r/20210714162424.4412-3-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>