pcm: dmix: Fix the inconsistent PCM state

The commit 38a2d2ed ("pcm: dmix: Do not discard slave reported
delay in...") changed the handling in snd_pcm_dmix_status() for taking
the actual delay from the slave PCM status.  Along with it, the commit
removed the line to update its own state altogether, as it had been
done originally in the dshare patch (commit faf53c19 "pcm_dshare:
Do not discard slave reported delay..."), supposing that the slave PCM
keeps this same state.  However, for dmix/dshare, the PCM state may
differ from the slave, thus these changes resulted in the inconsistent
PCM state.

For dshare, the issue was already addressed by commit ad6957c6
("plugin:dshare: wrong state reporting"), while the fix for dmix was
forgotten until now.

This patch restores the code to set the proper dmix PCM state again
like in the previous versions.

Fixes: 38a2d2ed ("pcm: dmix: Do not discard slave reported delay in...")
Reported-by: Cheng Sun <chengsun9@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>