IB/core: Refactor GID cache's ib_dispatch_event

Refactor ib_dispatch_event into a new function in order to avoid
duplicating code in the next patch.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Reviewed-by: Haggai Eran <haggaie@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>