drm/radeon: fix support for vline relocations.

Userspace sends us a special relocation type to sync video/exa
to vlines to avoid tearing, this deals with the relocation
in the kernel, it picks the correct crtc and avoids issues
where crtcs are disabled.

This version also parses the wait until to make sure it isn't
trying to do anything evil.

Signed-off-by: Dave Airlie <airlied@redhat.com>