Remove last references of vm_ih

The only remaining user was busdma, and so it was simplified.

Reviewed by: markj, jhb
MFC after: 1 week
Fixes: 254e4e5b77d7 ("Simplify swi for bus_dma")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36936

(cherry picked from commit e23c3445f69c945a8d5316b26bd996dbc900b2aa)