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

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

Cc: stable@vger.kernel.org
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>

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;