sound: Standardize mixer_init() call order

[christos]

Summary

Device drivers call mixer_init() in a non-standard order - some before
pcm_init(), and some after pcm_register(). However, both approaches are
potentially risky, and logically weak, since pcm_register() is supposed
to be the function that finalizes sound(4) attach.

Standardize the call ordering by moving all mixer_init() calls after
pcm_init(). This is also necessary for a follow-up patch, which expects
pcm_init() to have run first and initialized the PCM lock.

Sponsored by: The FreeBSD Foundation
MFC after: 1 month