KVM: x86: Move update_cr8_intercept() to lapic.c

KVM: x86: Move update_cr8_intercept() to lapic.c

Move update_cr8_intercept() to lapic.c so that it's globally visible
in anticipation of extracting most of the register-specific code out of
x86.c and into a new compilation unit.  Opportunistically prefix the
helper kvm_lapic_ to make its role/scope more obvious.

No functional change intended.

Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Yosry Ahmed <yosry@kernel.org>
Link: https://patch.msgid.link/20260529222223.870923-14-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>