This patch introduces the ops table to each memory allocation type
(SNDRV_DMA_TYPE_XXX) and abstract the handling for the better code
management.  Then we get separate the page allocation, release and
other tasks for each type, especially for the SG buffer.

Each buffer type has now callbacks in the struct snd_malloc_ops, and
the common helper functions call those ops accordingly.  The former
inline code that is specific to SG-buffer is moved into the local
sgbuf.c, and we can simplify the PCM code without details of memory
handling.

Link: https://lore.kernel.org/r/20210609162551.7842-4-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>