drm/ttm: make ttm_tt unbind function return void.

The return value just led to BUG_ON, I think if a driver wants
to BUG_ON here it can do it itself. (don't BUG_ON).

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200728040003.20398-1-airlied@gmail.com