ASoC: bcm: Use managed PCM buffer allocation

As the standard buffer allocation helper supports WC pages now, we can
convert bcm driver to use that.  This allows us to remove lots of
superfluous code.

Acked-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210802072815.13551-7-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>