drm: Asynchronouse page flipping interface and Intel implementation

Here's a sequence of five patches that exposes an interface to request of the driver that the page flipping request be executed without waiting for vblank. It's optional, and drivers can expose whether it is supported through the existing GETCAP ioctl.

This supports only Ivybridge and Sandybridge Intel graphics chips as that's what I've got to test on; of course all of the other drivers have been modified so that they still compile cleanly.

-- keith.packard@intel.com