drm/amdgpu: move ttm bo->offset to amdgpu_bo

GPU address should belong to driver not in memory management.
This patch moves ttm bo.offset and gpu_offset calculation to amdgpu driver.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/372930/