drm/amdgpu: Don't reserve vram as WC for A+A

On A+A platform, vram can be mapped as WB. Not necessarily
to always map vram as WC on such platform.

Calling function arch_io_reserve_memtype_wc will mark the
whole vram region as WC. So don't call it for A+A platform.

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Suggested-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian Konig <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>