sound/soc/pxa/mmp-pcm.c: use gen_pool_dma_alloc() to allocate dma buffer

sound/soc/pxa/mmp-pcm.c: use gen_pool_dma_alloc() to allocate dma buffer

Since gen_pool_dma_alloc() is introduced, we implement it to simplify code.

Signed-off-by: Nicolin Chen <b42378@freescale.com>
Cc: Eric Miao <eric.y.miao@gmail.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>