Support from vblank events in drm.

Support from vblank events in drm.

Previously, if userland wanted to wait on a certain vertical blank, it
had to call an ioctl which slept. Now, they can ask for an even on the
drm fd, which is then read off, and can be poll(4)ed on. For dri2 this
fits better into the workflow since the fd gets added to the xserver
main loop, and replies to the dri2 clients happen upon recieving the
events.

This functionality is only used with xserver 1.8 (and for the intel
driver in our tree, this support is currently #if 0ed out due to bugs
with vblanks on 945 that are still being chased)

matthieu@ ok.