drm/ttm: add bo wait that takes a ctx wrapper.

I'm thinking of pushing the wait into the drivers.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200923030454.362731-6-airlied@gmail.com