drm/amdgpu: revert "stop using gart_start as offset for the GTT domain"

Turned out the commit is incomplete and since we remove using the AGP
mapping from the GTT manager it is also not necessary any more.

This reverts commit 22d8bfaf.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: James Zhu <James.Zhu@amd.com>
Tested-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>