pcm: dmix: Fix semaphore usage with lockless operation

pcm: dmix: Fix semaphore usage with lockless operation

As Maarten Baert recently reported, the current dmix code applies the
semaphore unnecessarily around mixing streams even when the lockless
mix operation is used on x86.  This was rather introduced mistakenly
at the commit 267d7c728196 ("Add support of little-endian on
i386/x86_64 dmix") where the generic dmix code was included on x86,
too.

For achieving the original performance back, this patch changes the
semaphore handling to be checked at run time instead of statically at
compile time.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>