Graphics processing hang via recursive fence locking

Improper locking in the PowerVR DRM scheduler in Linux allows local users to hang graphics processing through fence release. The fence release path runs scheduler-queue cleanup synchronously while a DMA-BUF reservation mutex is held, and pvr_gem_object_vunmap then tries to acquire another mutex of the same class. Reaching the flaw requires local low-privileged access to the affected PowerVR DRM path; the resulting recursive lock can deadlock graphics work without affecting confidentiality or integrity.