MFC r258262:

drm: Support DRM_CAP_TIMESTAMP_MONOTONIC capability

This fixes DPMS with KDE and radeonkms. Without this, the display would
freeze when the monitor is put into sleep state, and only resumes after
several dozens of minutes once the monitor is powered on again.

Tested by:	Mathias Picker <Mathias.Picker@virtual-earth.de>
Approved by:	re (kib)