X86: disabled screensaver (dpms)

Please make sure your MR is ready to be merged !

• You added the changes in CHANGELOG.md
• You described the MR as below
• You updated the recalbox wiki
• You wrote a blog post on recalbox-blog

Changes :

• added xset in x86/x86_64 defconfig
• disabled dpms to prevent screen to turns blank

Closes #180 (closed)