drm/amdgpu: Use physical translation mode to access page table

On A+A platform, CPU write page directory and page table in cached
mode. So it is necessary for page table walker to snoop CPU cache.
This setting is necessary for page walker to snoop page directory
and page table data out of CPU cache.

Signed-off-by: Oak Zeng <Oak.Zeng@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>