drm/amd/amdgpu: Assign GART pages to AMD device mapping

This allows kernel mapped pages like the PDB and PTB to be
read via the iomem debugfs when there is no vram in the system.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org # 6.7.x