sound: Move sndstat_prepare_pcm() to pcm/sndstat.c and remove sndstat_entry->handler

Since all sndstat_entry->handler fields point to sndstat_prepare_pcm(),
we can just call the function directly, without assigning it to a
function pointer and calling it indirectly.

While here, move sndstat_prepare_pcm() to pcm/sndstat.c, as it is more
suitable there.

No functional change intended.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D44571