ASoC: meson: Use managed DMA buffer allocation

Using a managed buffer will pre-allocate the buffer using
snd_pcm_lib_preallocate_pages() and automatically free it when the PCM is
destroyed.

In addition it will call snd_pcm_lib_malloc_pages() before the driver's
hw_params() callback and snd_pcm_lib_free_pages() after the driver's
hw_free() callback.

This slightly reduces the boilerplate code of the driver.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Tested-by: Jerome Brunet <jbrunet@baylibre.com>