qxl: drop dummy functions

These days drm core checks function pointers everywhere before calling
them.  So we can drop a bunch of dummy functions now.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20180420071904.24276-5-kraxel@redhat.com