drm/ttm: drop bus.size from bus placement.

This is always calculated the same, and only used in a couple of places.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200811074658.58309-2-airlied@gmail.com