linux/common-config: Make backlight device support built-in

[zhaofengli]

The value of CONFIG_BACKLIGHT_CLASS_DEVICE must be the same as that of CONFIG_DRM, so that backlight devices can be properly attached to their respective DRM panels.

Arch also does the same.

Longer Explanation

In include/drm/drm_panel.h, drm_panel_of_backlight is a function that finds the corresponding backlight in the device tree and attaches it to the DRM panel. If CONFIG_DRM is built-in but CONFIG_BACKLIGHT_CLASS_DEVICE isn't, the function simply becomes noop:

#if IS_ENABLED(CONFIG_DRM_PANEL) && (IS_BUILTIN(CONFIG_BACKLIGHT_CLASS_DEVICE) || \
	(IS_MODULE(CONFIG_DRM) && IS_MODULE(CONFIG_BACKLIGHT_CLASS_DEVICE)))
int drm_panel_of_backlight(struct drm_panel *panel);
#else
static inline int drm_panel_of_backlight(struct drm_panel *panel)
{
	return 0;
}
#endif

In a broken setup, if the backlight isn't powered on already before the kernel is started, setting the brightness from userspace has no effect since backlight_get_brightness will be hard-wired to return 0. Depending on the hardware, there can be no way to enable the backlight (thus "unblanking" it) from userspace.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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