drm/amdgpu: Avoid direct cast to amdgpu_ttm_tt

For typesafety, use container_of() instead of implicit cast from struct
ttm_tt to struct amdgpu_ttm_tt.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>