RadeonHD: Bit-bang LVDS for edid

* Older cards have to use the LVDS_Info table,
  newer cards also have an ddc pin for EDID.
* Some buggy sbios don't inject the EDID into
  the vbios as they should
* This corrects hrev45812 as we need to still
  call connector_read_mode_lvds to obtain the
  spread spectrum data for the lvds.
* Call connector_read_mode_lvds, bit-bang the
  connector, choose the best outcome.