The linux function does a lot more than that as multiple waitqueue could be fetch
from a static table based on the hash of the argument but since in DRM it's only used
in one place just add a single variable.
We will probably need to change that in the futur but it's ok with DRM even with current
linux.

Reviewed by:	hselasky
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D26054