drm/i915: Fix oops in overlay due to frontbuffer tracking

commit 9169757a upstream.

The vma will be NULL if the overlay was previously off, so
dereferencing it will oops. Check for NULL before doing that.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Fixes: 9b3b7841 ("drm/i915/overlay: Use VMA as the primary tracker for images")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1481131693-27993-2-git-send-email-ville.syrjala@linux.intel.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
(cherry picked from commit 4a15cdbb)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/i915/intel_overlay.c

@@ -840,8 +840,8 @@ static int intel_overlay_do_put_image(struct intel_overlay *overlay,
 	if (ret)
 		goto out_unpin;
 
-	i915_gem_track_fb(overlay->vma->obj, new_bo,
-			  INTEL_FRONTBUFFER_OVERLAY(pipe));
+	i915_gem_track_fb(overlay->vma ? overlay->vma->obj : NULL,
+			  vma->obj, INTEL_FRONTBUFFER_OVERLAY(pipe));
 
 	overlay->old_vma = overlay->vma;
 	overlay->vma = vma;