drm/ttm: remove bdev from ttm_tt

I want to split this structure up and use it differently,
step one remove bdev pointer from it and pass it explicitly.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826014428.828392-4-airlied@gmail.com