alpha: fix ALSA DMA mmap crash

Make dma_alloc_coherent respect gfp flags (__GFP_COMP is one that
matters).

Signed-off-by: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Tested-by: Michael Cree <mcree@orcon.net.nz>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Jaroslav Kysela <perex@perex.cz>
Cc: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>