Import the drm_global references helpers.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 month
Import the preliminary port of the TTM.

The early commit is done to facilitate the off-tree work on the
porting of the Radeon driver.

Sponsored by:	The FreeBSD Foundation
Debugged and tested by:	    dumbbell
MFC after:	1 month
Add a generic way to call per event allocate / release function.

Reviewed by:	mav
MFC after:	1 month