ALSA: pcm: Avoid mmap warnings on x86

On x86, using dma_mmap_coherent() for the pages allocated via
dma_alloc_coherent() results in a warning like:
  aplay:32536 map pfn RAM range req uncached-minus for [mem 0x21d500000-0x21d51ffff], got write-back

Until the issue is addressed in the core side, take back to the old
good way in PCM code only for x86.

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