sound: Merge pcm/matrix_map.h with pcm/matrix.h

No functional change intended.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D47737

(cherry picked from commit 5a217a8d7d2a0dc19eb5d7bb1bd0f21116fbcf69)