drm/fbdev: Update legacy plane->fb refcounting for atomic restore

Starting with commit

        commit 28cc504e
        Author: Rob Clark <robdclark@gmail.com>
        Date:   Tue Aug 25 15:36:00 2015 -0400

            drm/i915: enable atomic fb-helper

I've been seeing some panics on i915 when the DRM master shuts down that appear
to be caused by using an already-freed framebuffer (i.e., we're unexpectedly
dropping our initial FB's reference count to 0 and freeing it, which causes a
crash when we try to restore it later).  Digging deeper, the state FB
refcounting is working as expected, but we seem to be missing proper
refcounting on the legacy plane->fb pointers in the new atomic fbdev code.

Tracking plane->old_fb and then doing a ref/unref at the end of the
fbdev restore like we do in the legacy ioctl's ensures we don't miscount
references on plane->fb and avoids the panics.

v2 from Daniel:

Really do what the atomic ioctl does:
- Also update plane->fb and plane->crtc.
- Clear out plane->old_fb on failures too.

v3: git add everything. Oops.

v4: Also clear old_fb in all other failure paths, spotted by David.

Cc: Rob Clark <robdclark@gmail.com>
Cc: intel-gfx@lists.freedesktop.org
Cc: David Herrmann <dh.herrmann@gmail.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com> (v1)
Reviewd-by: NDavid Herrmann <dh.herrmann@gmail.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_fb_helper.c

@@ -352,6 +352,8 @@ static int restore_fbdev_mode_atomic(struct drm_fb_helper *fb_helper)
 	drm_for_each_plane(plane, dev) {
 		struct drm_plane_state *plane_state;
 
+		plane->old_fb = plane->fb;
+
 		plane_state = drm_atomic_get_plane_state(state, plane);
 		if (IS_ERR(plane_state)) {
 			ret = PTR_ERR(plane_state);
@@ -382,16 +384,27 @@ static int restore_fbdev_mode_atomic(struct drm_fb_helper *fb_helper)
 	}
 
 	ret = drm_atomic_commit(state);
-	if (ret != 0)
-		goto fail;
-
-	return 0;
 
 fail:
+	drm_for_each_plane(plane, dev) {
+		if (ret == 0) {
+			struct drm_framebuffer *new_fb = plane->state->fb;
+			if (new_fb)
+				drm_framebuffer_reference(new_fb);
+			plane->fb = new_fb;
+			plane->crtc = plane->state->crtc;
+
+			if (plane->old_fb)
+				drm_framebuffer_unreference(plane->old_fb);
+		}
+		plane->old_fb = NULL;
+	}
+
 	if (ret == -EDEADLK)
 		goto backoff;
 
-	drm_atomic_state_free(state);
+	if (ret != 0)
+		drm_atomic_state_free(state);
 
 	return ret;