pcm: dmix: Handle slave PCM xrun and unexpected states properly

pcm: dmix: Handle slave PCM xrun and unexpected states properly

Currently, dmix & co plugins ignore the XRUN state of the slave PCM.
It's (supposedly) because dmix deals with the PCM in a free-wheel
mode, which is equivalent with XRUN.  But, this difference (whether
the correct freewheel or XRUN) should be done by the kernel, and we
may have an XRUN state indeed (e.g. via xrun injection).

This patch fixes this lack of behavior, to handle PCM xrun and does
prepare when the slave PCM is in such a state.

Also, the patch consolidates the prepare callback for all dmix, dsnoop
and dshare plugins, and fix/cleanup a bit for dshare/dsnoop codes to
align with dsnoop code.

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