drm/i915: Wait for page_sizes_gtt in gtt selftest on CHV/BXT+VTD

drm/i915: Wait for page_sizes_gtt in gtt selftest on CHV/BXT+VTD

VMA pinning to GGTT is now commited asynchronously in CHV / BXT+VDT
environments to avoid lock inversion among reservation_ww and cpu_hotplug
locks, the latter acquired from stop_machine().  Then,
vma->resource->page_sizes_gtt the test uses as shift count may still be
not populated (equal 0) after i915_vma_pin() returns.  Wait for VMA bind
completion in those cases to avoid shift-out-of-bounds kernel warnings and
the test case failing with -EBADSLT.

v2: Explain why VMA pinning is commited asynchronously on CHV/BXT+VTD
    (Krzysztof),
  - use more precise wording in commit description.

Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Reviewed-by: Sebastian Brzezinka <sebastian.brzezinka@intel.com>
Reviewed-by: Krzysztof Karas <krzysztof.karas@intel.com>
Acked-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://lore.kernel.org/r/20251023082925.351307-8-janusz.krzysztofik@linux.intel.com