linuxkpi: Add irq_work.h

Since handlers are call in a thread context we can simply use a workqueue
to emulate those functions.
The DRM code was patched to do that already, having it in linuxkpi allows us
to not patch the upstream code.

Sponsored-by: The FreeBSD Foundation
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D24859