Very sorry to hear that your LCD backlight problem is not resolved yet!
Are you confident that LCD backlight is not faulty, and have you tried reinstalling coreboot?
If answer for both questions is "yes" - then you could try these two advices:
1) Re-download a source code of coreboot from github, compile it and flash again to your G505s.
Since your last try - at the end of January - there were 5 commits to "coreboot/src/mainboard/lenovo/g505s" branch,
and also some commits to the common branches, changes that affect all the mainboards supported by coreboot.
Maybe your issue will be resolved if you would try coreboot again, with latest sources!
2) If 1st advice doesn't work, you could open "coreboot/src/mainboard/lenovo/g505s/buildOpts.c" file
(which is the only file from G505s branch that has a reference to "LCD")
Go to line 158, try to change the "#define BLDCFG_CFG_LCD_BACK_LIGHT_CONTROL 200" parameter.
Default value seems to be 200 for most platforms, but you could try setting it to 0. Then compile & flash again
=== Any advices from experienced coreboot members would be very welcome! ===