drm/amdgpu: Only retrieve GPU address of GART table after pinning it

Doing it earlier hits a WARN_ON_ONCE in amdgpu_bo_gpu_offset.

Fixes: "drm/amdgpu: remove gart.table_addr"

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>