glx: guard glx_screen frontend_screen member

Guards workaround code with the same conditions as glx_screen`s
frontend_screen member.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

Fixes: 67eeee43e0 ("driconf: add a way to override GLX_CONTEXT_RESET_ISOLATION_BIT_ARB")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/15021
(cherry picked from commit bd42f62b0f)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/40359>

.pick_status.json

@@ -934,7 +934,7 @@
         "description": "glx: guard glx_screen frontend_screen member",
         "nominated": true,
         "nomination_type": 2,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "67eeee43e07ddd3ba56fd546468dfd0c570af74c",
         "notes": null

src/glx/create_context.c

@@ -98,10 +98,11 @@ glXCreateContextAttribsARB(Display *dpy, GLXFBConfig config,
       attrib_list = malloc(sizeof(int) * num_attribs * 2);
 
       uint8_t clear_ctx_reset_isolation_bit = false;
+#if defined(GLX_DIRECT_RENDERING) && (!defined(GLX_USE_APPLEGL) || defined(GLX_USE_APPLE))
       dri2GalliumConfigQueryb(psc->frontend_screen,
                               "glx_clear_context_reset_isolation_bit",
                               &clear_ctx_reset_isolation_bit);
-
+#endif
       for (unsigned i = 0; i < num_attribs; i++) {
          attrib_list[i * 2] = orig_attrib_list[i * 2];
          if (clear_ctx_reset_isolation_bit &&