Your best solution is probably to write an EDID blob (or reuse one you find
somewhere) that provides at least one mode matching your TV's native mode
(probably full-HD).
Google suggested the following document:
http://www.jordansmanuals.com/ServiceManuals%5CLG%5CTV%5CLCD%5C42LB9DF%5C42LB9DF%20Service%20Manual.pdf
which on page 13/14 shows the full EDID blob for the various HDMI outputs of the
TV. You may want to read that document, convert the EDID blobs to 512 bytes binary
files and hell DRM core to use the right one via module/kernel cmdline option:
drm_kms_helper.edid_firmware=edid/lg42lb9df.edid
or
modprobe drm-kms-helper edid_firmware=edid/lg42lb9df.edid
where
/lib/firmware/edid/lg42lb9df.edid
is the 512-bytes EDID blob created according to data from above manual.
(note, that will only work for intel, radeon and nouveau drivers but will
not work for closed drivers of AMD/nVidia)