drm: connector->dpms is not optional

We always register the DPMS property, it's really a fundamental
part of a display driver. So don't check whether the vfunc is there,
it's non-optional

Yes I've audited all the almost 100 drm_connector_funcs we have, no
one botched this ;-)

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449218769-16577-19-git-send-email-daniel.vetter@ffwll.ch


Reviewed-by: Thierry Reding <treding@nvidia.com>