drm/radeon/kms: Skip ACPI call to ATIF when possible

I am under the impression that it only makes sense to call the ATIF
method if the graphics device has an ACPI handle attached. So we could
skip the call altogether if there is no such handle.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>