This is an attempt to make the previous fix a bit more robust going
forward.

v2:
* Only allow DRM_CAP_TIMESTAMP_MONOTONIC with UMS drivers (Daniel
  Vetter, Alex Deucher)
* Different logic to keep DRM_CAP_TIMESTAMP_MONOTONIC separate from
  the other caps (Daniel Vetter)
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161201073731.5716-1-michel@daenzer.net