Update to DRM git.

Update to DRM git.

Some stability fixes for radeon. The most part of this diff is related
to fixing up the VBLANK (vertical blank interrupt) handling. Now, if the
X driver supports the DRM_IOCTL_MODESET_CTL ioctl, (to be used when
changing the video modes), then allow the vblank to be disabled once
that ioctl has been called. Otherwise, keep the interrupt enabled at all
time, since disabling it otherwise will lead to problems.

Tested by a few. "no problem" on API/ABI deraadt@.