mi: Fix infinite loop on regen when swrast_dri.so is missing

The swrast DRI provider gets pushed on the glx provider stack at every
server generation, so the stack turns into a circular list on regen.

X.Org bug#18388 <https://bugs.freedesktop.org/show_bug.cgi?id=18388>
(cherry picked from commit d3d6be4948)

mi/miinitext.c

@@ -661,7 +661,8 @@ InitExtensions(argc, argv)
 #endif
 
 #ifdef GLXEXT
-    GlxPushProvider(&__glXDRISWRastProvider);
+    if (serverGeneration == 1)
+	GlxPushProvider(&__glXDRISWRastProvider);
     if (!noGlxExtension) GlxExtensionInit();
 #endif
 }