drm/virtio: Drop dummy gamma table support

No need to confuse userspace like this.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459331485-28376-8-git-send-email-daniel.vetter@ffwll.ch