Revert "kvm: x86: optimize dr6 restore"

This reverts commit 0e0a53c5.

As Christian Ehrhardt noted:

  The most common case is that vcpu->arch.dr6 and the host's %dr6 value
  are not related at all because ->switch_db_regs is zero. To do this
  all correctly, we must handle the case where the guest leaves an arbitrary
  unused value in vcpu->arch.dr6 before disabling breakpoints again.

  However, this means that vcpu->arch.dr6 is not suitable to detect the
  need for a %dr6 clear.

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>