ASoC: tegra: Use managed buffer allocation

As the standard buffer allocation helper supports WC pages now, we can
convert imx-pcm-rpmsg 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/20210802072815.13551-10-tiwai@suse.de


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