gicv3: Only set the redistributor base if we're not prealloced

Only set the redistributor base if we're not reallocated. If we are
preallocated, leave it all alone.

Sponsored by: Netflix
Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D44035

(cherry picked from commit ae84491a3594c542f2aedc9340581cbb30386c1e)