drm/ttm: merge offset and base in ttm_bus_placement

This is used by TTM to communicate the physical address
which should be used with ioremap(), ioremap_wc(). We don't
need to separate the base and offset in any way here.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/389457/