drm/i915/sdvo: Handle unsupported GET_SUPPORTED_ENHANCEMENTS gracefully

In the event that the external chipset doesn't implement the
GET_SUPPORTED_ENHANCEMENTS commands, gracefully treat it as having no
enhancments rather than bailing.

Reported-and-tested-by: Woody Suwalski <terraluna977@gmail.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=18342
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>