drm: qxl: Remove unused CRTC .dpms() helper operation

The CRTC .dpms() helper operation is called by the atomic helpers only
when no .prepare(), .atomic_disable() or .disable() operation is
provided. As the qxl driver provides a .disable() operation, the .dpms()
operation is unused and can be removed.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170627211621.27767-3-laurent.pinchart+renesas@ideasonboard.com