arm64: KVM: Switch vgic save/restore to alternative_insn

So far, we configured the world-switch by having a small array
of pointers to the save and restore functions, depending on the
GIC used on the platform.

Loading these values each time is a bit silly (they never change),
and it makes sense to rely on the instruction patching instead.

This leads to a nice cleanup of the code.

Acked-by: Will Deacon <will.deacon@arm.com>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

virt/kvm/arm/vgic.c

@@ -2128,9 +2128,6 @@ int kvm_vgic_hyp_init(void)
 		goto out_free_irq;
 	}
 
-	/* Callback into for arch code for setup */
-	vgic_arch_setup(vgic);
-
 	on_each_cpu(vgic_init_maintenance_interrupt, NULL, 1);
 
 	return 0;