drm/nv50-nvc0: activate/update ds channel's framebuffer on modesets

The hw doesn't really appear to be designed to be used the way we have to
use it due to DRI2's design.  This leads us to having to keep the flipped
fb support active at all times.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nv50_crtc.c

@@ -487,6 +487,7 @@ nv50_crtc_prepare(struct drm_crtc *crtc)
 
 	NV_DEBUG_KMS(dev, "index %d\n", nv_crtc->index);
 
+	nv50_display_flip_stop(crtc);
 	drm_vblank_pre_modeset(dev, nv_crtc->index);
 	nv50_crtc_blank(nv_crtc, true);
 }
@@ -502,6 +503,7 @@ nv50_crtc_commit(struct drm_crtc *crtc)
 	nv50_crtc_blank(nv_crtc, false);
 	drm_vblank_post_modeset(dev, nv_crtc->index);
 	nv50_crtc_wait_complete(crtc);
+	nv50_display_flip_next(crtc, crtc->fb, NULL);
 }
 
 static bool
@@ -683,11 +685,16 @@ nv50_crtc_mode_set_base(struct drm_crtc *crtc, int x, int y,
 {
 	int ret;
 
+	nv50_display_flip_stop(crtc);
 	ret = nv50_crtc_do_mode_set_base(crtc, old_fb, x, y, false);
 	if (ret)
 		return ret;
 
-	return nv50_crtc_wait_complete(crtc);
+	ret = nv50_crtc_wait_complete(crtc);
+	if (ret)
+		return ret;
+
+	return nv50_display_flip_next(crtc, crtc->fb, NULL);
 }
 
 static int
@@ -697,6 +704,7 @@ nv50_crtc_mode_set_base_atomic(struct drm_crtc *crtc,
 {
 	int ret;
 
+	nv50_display_flip_stop(crtc);
 	ret = nv50_crtc_do_mode_set_base(crtc, fb, x, y, true);
 	if (ret)
 		return ret;