glx: Compute the GLX extension string from __glXScreenInit

Now that the enable bits are in the screen base class we can compute
this in one place, rather than making every backend do it.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>

glx/glxscreens.c

@@ -383,6 +383,14 @@ __glXScreenInit(__GLXscreen * pGlxScreen, ScreenPtr pScreen)
     }
 
     dixSetPrivate(&pScreen->devPrivates, glxScreenPrivateKey, pGlxScreen);
+
+    i = __glXGetExtensionString(pGlxScreen->glx_enable_bits, NULL);
+    if (i > 0) {
+        pGlxScreen->GLXextensions = xnfalloc(i);
+        (void) __glXGetExtensionString(pGlxScreen->glx_enable_bits,
+                                       pGlxScreen->GLXextensions);
+    }
+
 }
 
 void