drm/gma500/mrst: Don't blindly guess a mode for LVDS

Previously we always had something hooked up to LVDS so we tried very
hard to get a mode. With the Minnowboard this is no longer the case.
If no mode can be found over DDC or the firmware we just ignore LVDS.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>