Great news! /u/QubesN00b flashed this dGPU build to his "no-dGPU"
G505S and everything seems to be working fine for him :D (
https://www.reddit.com/r/coreboot/comments/ar8v7d/if_your_g505s_does_not_have_a_discrete_gpu/
) . Now I could e.g. remove "if
(IS_ENABLED(CONFIG_MULTIPLE_VGA_ADAPTERS))" condition from
https://review.coreboot.org/c/coreboot/+/31450/3/src/mainboard/lenovo/g505s/OemCustomize.c
- and set BottomIo to 0xD0 for all G505S.