drm/amdgpu: move GART recovery into GTT manager v2

The GTT manager handles the GART address space anyway, so it is
completely pointless to keep the same information around twice.

v2: rebased

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>