xf86-video-modesetting: Don't enable UNIVERSAL_PLANES separately

We don't want universal_planes unless we're using atomic APIs for
modesetting, and the kernel already enables universal_planes
automatically when atomic is enabled.

If we enable universal_planes when we're not using atomic, then we
won't have selected a plane for each crtc, and this will break lease
creation which requires planes for each output when universal_planes
is enabled.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>