drm/ttm: split bound/populated flags.

Move bound up into the bo object, and keep populated with the tt
object.

The ghost object handling needs to follow the flags at the bo
level now instead of it being part of the ttm tt object.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200915024007.67163-7-airlied@gmail.com