arm64/vgic_v3: Fix an inverted test when reading GICD_I<C|S>ENABLER

On read, these registers' fields return 1 if forwarding of the
corresponding interrupt is enabled, and 0 otherwise. The test in
read_enabler() was inverted.

Reported by: Kevin Day <kevin@your.org>
Reviewed by: andrew
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D54678