ALSA: memalloc: Fix pgprot for WC mmap on x86

We have a special handling of WC pages on x86, and it's currently
specific to HD-audio.  The last forgotten piece was the pgprot setup
for the mmap with WC pages.

This patch moves the pgprot setup for WC pages from HD-audio-specific
mmap callback to the common helper code.  It allows us to remove the
superfluous mmap callback in HD-audio and its prepare_mmap
redirection.

Link: https://lore.kernel.org/r/20210804061329.29265-1-tiwai@suse.de


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