CVE-2026-68247

In the Linux kernel, the following vulnerability has been resolved:

drm/i915/bios: range check LFP Data Block panel_type2

While the panel_type from LFP Data Block is range checked, panel_type2
is not. Add a few helpers for range checking, and use them to not only
check panel_type2, but also improve clarity and correctness in the panel
type selection.

Discovered using AI-assisted static analysis confirmed by Intel Product
Security.

v2:
- Fix commit message typo (Michał)
- Add is_panel_type_pnp() (Ville)

(cherry picked from commit c9ebe5d2f25729d6cfbbb1235d640bf67f9275df)