Use modesetting driver for installation on Matrox in secure boot mode.

fbdev driver doesn't work on top of mgag200 KMS driver (fbdev assumes
32bpp in 24bit color depth, but mga uses 24bpp instead). As workaround
use modesetting driver already during installation. (bnc#814587)
startup/arch/x86_64/x11.sh: $XServer needs to be overwritten

$XServer needs to be overwritten, not $XServerAccel!
Added changelog entry and bumped version number to 2.17.106.