drm/i915: Fix up mismerge of 3490ea5d in 3.7.y

The 3.7.y version of this seems to have missed a hunk in i9xx_update_wm.

Tested-by: Glen Gray <slaine@slaine.org>
Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>