gallium: add PIPE_CAP_CONDITIONAL_RENDER

We were checking whether render_condition is set. That was not reliable,
because it's always set with trace and noop regardless of driver support.

Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/state_tracker/st_extensions.c

@@ -501,7 +501,7 @@ void st_init_extensions(struct st_context *st)
       ctx->Extensions.ARB_framebuffer_object = GL_TRUE;
    }
 
-   if (st->pipe->render_condition) {
+   if (screen->get_param(screen, PIPE_CAP_CONDITIONAL_RENDER)) {
       ctx->Extensions.NV_conditional_render = GL_TRUE;
    }