st/mesa: set geometry shader to NULL when doing internal drawing

The code expects the geometry shader to be NULL.
We don't have geometry shaders now, but it's good to be prepared.

v2: check for support in the cso context

src/mesa/state_tracker/st_extensions.c

@@ -555,7 +555,8 @@ void st_init_extensions(struct st_context *st)
       ctx->Extensions.ARB_vertex_type_2_10_10_10_rev = GL_TRUE;
    }
 
-   if (screen->get_shader_param(screen, PIPE_SHADER_GEOMETRY, PIPE_SHADER_CAP_MAX_INSTRUCTIONS) > 0) {
+   if (screen->get_shader_param(screen, PIPE_SHADER_GEOMETRY,
+                                PIPE_SHADER_CAP_MAX_INSTRUCTIONS) > 0) {
 #if 0 /* XXX re-enable when GLSL compiler again supports geometry shaders */
       ctx->Extensions.ARB_geometry_shader4 = GL_TRUE;
 #endif