CVE-2026-99090: drm/amdgpu: serialize vblank counter reads against GPU reset

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

drm/amdgpu: serialize vblank counter reads against GPU reset

The vblank counter callback reads display registers and scanout position without holding resetdomain->sem. During GPU recovery, display commit work can call the callback while reset owns the semaphore for writing. The hardware access then triggers the lockdepassertheld() in amdgpudeviceskiphwaccess() and sets TAINTWARN when lockdep is enabled.

Try to acquire the reset semaphore for reading before accessing the hardware, and hold it across the counter and scanout-position reads. If reset is already in progress, return DRM's cached software vblank count instead. This avoids blocking behind reset and avoids accessing display hardware while it is being reset.

The cached count is less accurate than the hardware count, but is used only when reset prevents a safe hardware read. This addresses the reset-time warning from this vblank path; it does not suppress or fix unrelated lockdep warnings.

(cherry picked from commit 51a4359f7431a4be5e42803c8bdaf7e0e5be2934)