gicv3: Don't allocate pend_base if we're already started

If the gicv3 is already started, then don't allocate memory for the
pend_base tables.

Sponsored by: Netflix
Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D44034

(cherry picked from commit ddd98f5a626df5de51fd5c2460592d9b95a49179)