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>

drivers/gpu/drm/i915/intel_acpi.c

@@ -208,7 +208,7 @@ static bool intel_dsm_pci_probe(struct pci_dev *pdev)
 
 	ret = intel_dsm(dhandle, INTEL_DSM_FN_SUPPORTED_FUNCTIONS, 0);
 	if (ret < 0) {
-		DRM_ERROR("failed to get supported _DSM functions\n");
+		DRM_DEBUG_KMS("failed to get supported _DSM functions\n");
 		return false;
 	}