gicv3_its: Leave LPI interrupts enabled during handling

This follows the behavior on x86 where edge triggered interrupts are
not disabled when executing the handler. Because the ITS is a shared
resource, contention for the command queue lock can be substantial.

Suggested by: gallatin
Reviewed by: andrew
Tested by: gallatin
Sponsored by: Ampere Computing LLC
Differential Revision: https://reviews.freebsd.org/D28709