drm/vgem fixes and new ioctl for testing prime

[Chris Wilson]

Just a quick resend of the existing vgem patches, all 3 have been acked,
but only the first 2 have reviews. The third involves both new ioctl and
dma-buf/fences, so perhaps people have been reluctant... But now is a
good time! These patches are exercised by intel-gpu-tools (or will be once
the new ioctls are ratified).
-Chris