drm: hide legacy drivers with CONFIG_DRM_LEGACY

Lets move forward and hide the remaining DRI1 drivers behind a config
option, so we have a central place to disable them all. Furthermore, we
can provide a clear warning to anyone enabling them.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20160804080657.9664-1-dh.herrmann@gmail.com