agp: correct missing cleanup on error in agp_add_bridge

While investigating a kmemleak detected leak, I encountered the
agp_add_bridge function.  It appears to be responsible for freeing
the agp_bridge_data in the case of a failure, but it is only doing
so for some errors.

Fix it to always free the bridge data if a failure condition is
encountered.

Signed-off-by: Kevin Winchester <kjwinchester@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>