KVM: arm64: Get sys_reg encoding from descriptor in idregs_debug_show()

KVM is about to add support for more VM-scoped feature ID regs that
live outside of the id_regs[] array, which means the index of the
debugfs iterator may not actually be an index into the array.

Prepare by getting the sys_reg encoding from the descriptor itself.
Reviewed-by: Sebastian Ott <sebott@redhat.com>
Link: https://lore.kernel.org/r/20240619174036.483943-2-oliver.upton@linux.devSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/sys_regs.c

@@ -3502,7 +3502,7 @@ static int idregs_debug_show(struct seq_file *s, void *v)
 		return 0;
 
 	seq_printf(s, "%20s:\t%016llx\n",
-		   desc->name, IDREG(kvm, IDX_IDREG(kvm->arch.idreg_debugfs_iter)));
+		   desc->name, IDREG(kvm, reg_to_encoding(desc)));
 
 	return 0;
 }