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.

Cc: Nicolin Chen <nicoleotsuka@gmail.com>
Cc: Xiubo Li <Xiubo.Lee@gmail.com>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Shengjiu Wang <shengjiu.wang@gmail.com>
Acked-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210802072815.13551-9-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>