radv: Add X11 adaptive sync support based on dri options.

Adds adaptive_sync support in the WSI + driconf parsing in radv.

FWIW I think for xf86-video-amdgpu this still requires an X11 config option, so only people who opt in can get possible regressions from this.

So then the remaining question is: why do this in the WSI?

It has been suggested in another MR that the application sets this. However, I disagree with that as I don't think we'll ever get a reasonable set of applications setting it.

The next questions is whether this can be a layer. It definitely can be as implemented now. However I have some ideas for future improvements (e.g. show frames twice to increase fps range) that are more messy to do cleanly without WSI support.

Of course, most of the WSI could almost be a layer, but I think this still fits best in the WSI.