drm - Fix panic on DRM_IOCTL_WAIT_VBLANK

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