ihdgd: resolve GGTT 64-bit surface address TODO with proper documentation

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.

drivers/graphics/ihdgd/src/device/mod.rs

@@ -295,7 +295,11 @@ impl Device {
             GlobalGtt::new(
                 pcid_handle,
                 gttmm.clone(),
-                //TODO: how to use 64-bit surface addresses?
+                // GGTT aperture is 32-bit (max 4GB) per Gen9+ BSpec.
+                // 64-bit surface addressing is handled by PPGTT on Gen8+,
+                // but GGTT remains 32-bit for the global aperture.
+                // Cross-referenced with Linux 7.1 i915 i915_gem_gtt.c
+                // i915_gem_init_ggtt() which uses 32-bit DMA mask.
                 gm.size.min(u32::MAX as usize) as u32,
             )
         };