drm - Fix panic on DRM_IOCTL_WAIT_VBLANK
* DRM_IOCTL_WAIT_VBLANK -> drm_wait_vblank() -> DRM_WAIT_ON() -> panic
DRM_WAIT_ON() assumed that the device was locked and tried to unlock
and relock it around the tsleep. However, the ioctl paths that use
this function do not lock the device.
* Remove the unlock/relock sequence.
* Tested on haswell using ioctl call w/vblank.request.type =
_DRM_VBLANK_RELATIVE and vblank.request.sequence = 10. Crash reproduced
before fix, does not occur after fix.
Reported-by: arcade@b1t.name