gicv3: Panic if the gicv3 already running

Due to undefined behavior, it's impossible to re-program a gicv3 ITS
table once it's programmed once. Memory corruption happens otherwise.
Panic if we detect the LPI is already enabled.

Sponsored by: Netflix
Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D44033

(cherry picked from commit 51c57ca92ed3d0caf8f3e8c61345ac670d494901)