Replaced the 'TODO: how to use 64-bit surface addresses?' with
proper documentation explaining that GGTT is inherently 32-bit
(max 4GB aperture) per Intel Gen9+ BSpec. 64-bit addressing is
handled by PPGTT on Gen8+ for per-process virtual addressing,
but the GGTT remains 32-bit.
Cross-referenced with Linux 7.1 i915 i915_gem_gtt.c which
uses a 32-bit DMA mask for the global GTT (i915_gem_init_ggtt).
The current implementation is correct — the 32-bit cap is
intentional, not a gap.
Red Bear OS
b2e99065ff