gst-plugins-bad: depend on directfb instead of selecting it for directfb plugin

As discussed here:
http://lists.busybox.net/pipermail/buildroot/2015-September/139213.html



It makes more sense to use 'depends on' (and default to y) for a big
dependency like directfb.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>