KVM: x86: Revert kvm_x86_ops.mem_enc_ioctl() back to an OPTIONAL hook

KVM: x86: Revert kvm_x86_ops.mem_enc_ioctl() back to an OPTIONAL hook

Restore KVM's handling of a NULL kvm_x86_ops.mem_enc_ioctl, as the hook is
NULL on SVM when CONFIG_KVM_AMD_SEV=n, and TDX will soon follow suit.

  ------------[ cut here ]------------
  WARNING: CPU: 0 PID: 1 at arch/x86/include/asm/kvm-x86-ops.h:130 kvm_x86_vendor_init+0x178b/0x18e0
  Modules linked in:
  CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.15.0-rc2-dc1aead1a985-sink-vm #2 NONE
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:kvm_x86_vendor_init+0x178b/0x18e0
  Call Trace:
   <TASK>
   svm_init+0x2e/0x60
   do_one_initcall+0x56/0x290
   kernel_init_freeable+0x192/0x1e0
   kernel_init+0x16/0x130
   ret_from_fork+0x30/0x50
   ret_from_fork_asm+0x1a/0x30
   </TASK>
  ---[ end trace 0000000000000000 ]---

Opportunistically drop the superfluous curly braces.

Link: https://lore.kernel.org/all/20250318-vverma7-cleanup_x86_ops-v2-4-701e82d6b779@intel.com
Fixes: b2aaf38ced69 ("KVM: TDX: Add place holder for TDX VM specific mem_enc_op ioctl")
Link: https://lore.kernel.org/r/20250502203421.865686-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>