drm/radeon: don't use ttm bo->offset

Calculate GPU offset in radeon_bo_gpu_offset without depending on
bo->offset.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-and-tested-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/372935/