glx: Move server GLX vendor and version strings to glx_screen

These can in fact vary between screens.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10418>

src/glx/tests/clientinfo_unittest.cpp

@@ -43,8 +43,6 @@ public:
       this->next = 0;
       this->dpy = dpy;
       this->minorVersion = minor;
-      this->serverGLXvendor = 0;
-      this->serverGLXversion = 0;
       this->glXDrawHash = 0;
 
       this->screens = new glx_screen *[dpy->nscreens];