setup-xorg-base: don't install video DDX

modesetting provides superior compatibility and comparable performance on modern platforms. other distros such as debian/ubuntu [0] and fedora [1] switched to modesetting several years ago. users who require maximum xrender performance can manually install a ddx.

[0] https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/thread/AUPYPJMFJZMHPEKN24LBABZKIEOV4NU5/ [1] https://tjaalton.wordpress.com/2016/07/23/intel-graphics-gen4-and-newer-now-defaults-to-modesetting-driver-on-x/