drm: Only decouple the old_fb from the crtc is we call mode_set*

Otherwise when disabling the output we switch to the new fb (which is
likely NULL) and skip the call to mode_set -- leaking driver private
state on the old_fb.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=29857Reported-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dave Airlie <airlied@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_crtc_helper.c

@@ -637,13 +637,13 @@ int drm_crtc_helper_set_config(struct drm_mode_set *set)
 		mode_changed = true;
 
 	if (mode_changed) {
-		old_fb = set->crtc->fb;
-		set->crtc->fb = set->fb;
 		set->crtc->enabled = (set->mode != NULL);
 		if (set->mode != NULL) {
 			DRM_DEBUG_KMS("attempting to set mode from"
 					" userspace\n");
 			drm_mode_debug_printmodeline(set->mode);
+			old_fb = set->crtc->fb;
+			set->crtc->fb = set->fb;
 			if (!drm_crtc_helper_set_mode(set->crtc, set->mode,
 						      set->x, set->y,
 						      old_fb)) {