thunderbolt: Handle bandwidth allocation mode enablement notification

When the graphics side enables bandwidth allocation mode the DP IN
adapter sends notification to the connection manager about this.
Currently the handler misses this and tries to allocate 0 Mb/s that then
makes the graphics side to think the request failed.

Fix this by properly handling the enablement notification.

Fixes: 6ce35635 ("thunderbolt: Add support for DisplayPort bandwidth allocation mode")
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>