KVM: x86/pmu: Remove KVM's enumeration of Intel's architectural encodings

KVM: x86/pmu: Remove KVM's enumeration of Intel's architectural encodings

Drop KVM's enumeration of Intel's architectural event encodings, and
instead open code the three encodings (of which only two are real) that
KVM uses to emulate fixed counters.  Now that KVM doesn't incorrectly
enforce the availability of architectural encodings, there is no reason
for KVM to ever care about the encodings themselves, at least not in the
current format of an array indexed by the encoding's position in CPUID.

Opportunistically add a comment to explain why KVM cares about eventsel
values for fixed counters.

Suggested-by: Jim Mattson <jmattson@google.com>
Tested-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Link: https://lore.kernel.org/r/20240109230250.424295-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>