drm: Import linux/timer.h from FreeBSD's OFED stack
linux/timer.h: Adapt to Dragonfly

* Protect callout structures from concurrent operations on MP machines

* Expand the API for use with drm code from Linux 3.8.x
drm: Use the Linux timer API

Reduce differences with Linux 3.8.13
drm/i915: Use the Linux timer API

Reduce differences with Linux 3.8.13