drm/i915: ignore lvds on AOpen Mini PC MP-915

This motherboard thinks it has an LVDS connected, so without this
patch the screen goes blank on the connected VGA monitor. More
information (for the non-KMS case) in fd.o bug #18004.

Signed-off-by: Tormod Volden <debian.tormod@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>