[coreboot] T400 screen issue / EDID handling

[Pi Van Den Cirkel]

> Dear coreboot community,
>
> tty0_ in #coreboot/#libreboot found a partial fix.
>
> See:
> http://libreboot.org/docs/hcl/gm45_lcd.html

Setting  gfx.lvds_dual_channel = 1 is a workaround so that the kernel
driver (i915) pickups the right value that coreboot had set. But there
are few things to be fixed:

1) No video (fb) during payload(grub) for gm45 (T500) with lcd with
dual channel lvds, up to the point where the kernel installs the i915
driver at which point i get working screen. (For single channel lvds
controllers all works ok - there is video during payload).

2) There are devices with same model that come both variants (i.e
T500) with a single or dual channel lvds . I dont know if there is
(currently) way in coreboot (without using the vbios blob) to detect
this dynamically (in runtime) instead of setting it in the devicetree.

I am willing to work on this problems, but my knowledge is limited in
this area. So, any hints on how to proceed, links to specs/docs, etc
are welcome.