drm/radeon/kms: make sure LVDS panel is valid in detect()

If the panel data is bogus this can lead to problems
later when the hardware trys to set the mode.  If the
data is invalid, report LVDS as disconnected.

Should fix fdo bug 24247.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>