alienware-wmi: make hdmi_mux enabled on case-by-case basis

Not all HW supporting WMAX method will support the HDMI mux feature.
Explicitly quirk the HW that does support it.

Signed-off-by: Mario Limonciello <mario_limonciello@dell.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>