[LightDM] Fixing "modprobe.d not a file" and "alternatives x86_64-linux-gnu_gfxcore_conf"

[Pander]

Hi all,

I am trying to find a fix for the problems reported at
https://bugs.launchpad.net/ubuntu/+source/ubuntu-drivers-common/+bug/1369216

In the last comment I have reported what I have tried by installing from
xenial proposed. For NVIDIA and ATI users there are many users that
experince this problem.

I'd like to help out find a fix. Please contact me for more information
and for testing.

Thanks,

Pander