[ARM] vgic: Minor performance improvements

This patch two performance improvements:
1. Treat all SGIs (i.e. irq < 16) as edge-triggered and
don't touch pending flag for edge-triggered interrupts
in __vgic_sync_vcpu_hwstate()
2. No need to test VGIC_TEST_ENABLED() along with
VGIC_TEST_PENDING() because an irq can only be pending
if it is enabled.

Signed-off-by: Anup Patel <anup@brainfault.org>