KVM: arm64: vgic: Use str_enabled_disabled() in vgic_v3_probe()

Remove hard-coded strings by using the str_enabled_disabled() helper
function.

Suggested-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250110225310.369980-2-thorsten.blum@linux.dev


Signed-off-by: Marc Zyngier <maz@kernel.org>