drm: Fix deadlock retry loop in page_flip_ioctl

I failed to properly onion-wrap the unwind code: We acquire the vblank
reference before we start with the wait-wound locking dance, hence we
must make sure we retry before we drop the reference. Oops.

v2: The vblank_put must be after the frambuffer_put (Michel). I suck at
unwrapping code that doesn't use separate labels for each stage, but
checks each pointer first ... While re-reading everything I also
realized that we must clean up the fb refcounts, and specifically
plane->old_fb before we drop the locks, either in the final unlocking,
or in the w/w retry path. Hence the correct fix is to drop the
vblank_put to the very bottom.

Fixes: 29dc0d1d ("drm: Roll out acquire context for the page_flip ioctl")
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Reported-by: Tommi Rantala <tt.rantala@gmail.com>
Cc: Tommi Rantala <tt.rantala@gmail.com>
Cc: Michel Dänzer <michel@daenzer.net>
Tested-by: Tommi Rantala <tt.rantala@gmail.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170522135945.28831-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_plane.c

@@ -948,8 +948,6 @@ int drm_mode_page_flip_ioctl(struct drm_device *dev,
 	}
 
 out:
-	if (ret && crtc->funcs->page_flip_target)
-		drm_crtc_vblank_put(crtc);
 	if (fb)
 		drm_framebuffer_put(fb);
 	if (crtc->primary->old_fb)
@@ -964,5 +962,8 @@ int drm_mode_page_flip_ioctl(struct drm_device *dev,
 	drm_modeset_drop_locks(&ctx);
 	drm_modeset_acquire_fini(&ctx);
 
+	if (ret && crtc->funcs->page_flip_target)
+		drm_crtc_vblank_put(crtc);
+
 	return ret;
 }