drm/i915: Silence _DSM errors

<@ajax> mjg59: how concerned should i be about [drm:intel_dsm_pci_probe]
        *ERROR* failed to get supported _DSM functions ?
<@mjg59> ajax: Entirely unconcerned

Signed-off-by: Adam Jackson <ajax@redhat.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>