Motivated by review comments from Ville&Sean.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Sean Paul <sean@poorly.run>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004202446.22905-21-daniel.vetter@ffwll.ch