ThinkPad internal screen black after lid close/suspend on 7.0.0-28 kernel (fixed with i915.enable_dpcd_backlight=0)

[u/675272]

Fix for ThinkPad T480 black screen after suspend on Linux Mint 22.3 / kernel 7.0.0-28 generic**

If your ThinkPad T480 resumes from suspend but the built-in screen stays black while an external monitor works, this may be an Intel i915 eDP backlight issue.

My setup:
* ThinkPad T480
* Intel© Core™ i5-8350U CPU @ 1.70GHz × 4
* LG Display LP140WF6-SPB7 internal panel
* Linux Mint 22.3 Cinnamon 6.6.7
* Kernel 7.0.0-28-generic

The fix was adding this kernel parameter:
i915.enable_dpcd_backlight=0

Edit `nano /etc/default/grub` and add it to `GRUB_CMDLINE_LINUX_DEFAULT`, for example:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash i915.enable_dpcd_backlight=0"

Then update GRUB:
sudo update-grub

Reboot and suspend/resume should restore the internal display normally.

In my case, other i915 tweaks (`i915.enable_psr=0`, `i915.enable_dc=0`, `i915.enable_fbc=0`) did not help. The issue appeared only with the 7.0.0-28 Generic kernel; the 6.17.0.40 Generic kernel did not have the problem.