ALSA: hda: Allow setting preallocation again for x86

ALSA: hda: Allow setting preallocation again for x86

The commit 9c699afeb873 ("ALSA: hda: No preallocation on x86
platforms") changed CONFIG_SND_HDA_PREALLOC_SIZE setup and its default
to zero for x86, as the preallocation should work almost all cases.
However, this expectation was too naive; some applications try to
allocate as the max buffer size as possible, and it leads to the
memory exhaustion.  More badly, the commit changed the kconfig no
longer adjustable for x86, so you can't fix it statically (although it
can be still adjusted via procfs).

So, practically seen, it's more recommended to set a reasonable limit
for x86, too.  This patch follows to that experience, and changes the
default to 2048 and allow the kconfig adjustable again.

Fixes: 9c699afeb873 ("ALSA: hda: No preallocation on x86 platforms")
Cc: <stable@vger.kernel.org>
BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=207223
Link: https://lore.kernel.org/r/20200413201919.24241-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>