st/dri: decrease input lag by syncing sooner in SwapBuffers

It's done by:
- decrease the number of frames in flight by 1
- flush before throttling in SwapBuffers
  (instead of wait-then-flush, do flush-then-wait)

The improvement is apparent with Unigine Heaven.

Previously:
    draw frame 2
    wait frame 0
    flush frame 2
    present frame 2

    The input lag is 2 frames.

Now:
    draw frame 2
    flush frame 2
    wait frame 1
    present frame 2

    The input lag is 1 frame. Flushing is done before waiting, because
    otherwise the device would be idle after waiting.

Nine is affected because it also uses the pipe cap.

src/gallium/auxiliary/util/u_screen.c

@@ -356,7 +356,7 @@ u_pipe_screen_get_param_defaults(struct pipe_screen *pscreen,
       return 0;
 
    case PIPE_CAP_MAX_FRAMES_IN_FLIGHT:
-      return 2;
+      return 1;
 
    case PIPE_CAP_DMABUF:
 #ifdef PIPE_OS_LINUX

src/gallium/state_trackers/dri/dri_drawable.c

@@ -562,19 +562,19 @@ dri_flush(__DRIcontext *cPriv,
        * flush method returns a fence even if there are no commands to flush.
        */
       struct pipe_screen *screen = drawable->screen->base.screen;
-      struct pipe_fence_handle *fence;
+      struct pipe_fence_handle *oldest_fence, *new_fence = NULL;
 
-      fence = swap_fences_pop_front(drawable);
-      if (fence) {
-         (void) screen->fence_finish(screen, NULL, fence, PIPE_TIMEOUT_INFINITE);
-         screen->fence_reference(screen, &fence, NULL);
+      st->flush(st, flush_flags, &new_fence);
+
+      oldest_fence = swap_fences_pop_front(drawable);
+      if (oldest_fence) {
+         screen->fence_finish(screen, NULL, oldest_fence, PIPE_TIMEOUT_INFINITE);
+         screen->fence_reference(screen, &oldest_fence, NULL);
       }
 
-      st->flush(st, flush_flags, &fence);
-
-      if (fence) {
-         swap_fences_push_back(drawable, fence);
-         screen->fence_reference(screen, &fence, NULL);
+      if (new_fence) {
+         swap_fences_push_back(drawable, new_fence);
+         screen->fence_reference(screen, &new_fence, NULL);
       }
    }
    else if (flags & (__DRI2_FLUSH_DRAWABLE | __DRI2_FLUSH_CONTEXT)) {