drm/ttm/tt: add wrappers to set tt state.

This adds 2 getters and 4 setters, however unbound and populated
are currently the same thing, this will change, it also drops
a BUG_ON that seems not that useful.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200915024007.67163-2-airlied@gmail.com