Wait a bit after killing the display manager to make sure the modules aren't still in use

While running the script to extract the VBIOS, I ran into an issue : the nvidia modules were seemingly still in use and modprobe couldn't remove them. After a bit of testing I found out that this problem did not arise when I waited a few seconds between stopping the display manager and removing the modules.

So this merge request is here to fix this problem for people who encounter it.