KVM: x86/pmu: Don't ignore bits 31:30 for RDPMC index on AMD

KVM: x86/pmu: Don't ignore bits 31:30 for RDPMC index on AMD

Stop stripping bits 31:30 prior to validating/consuming the RDPMC index on
AMD.  Per the APM's documentation of RDPMC, *values* greater than 27 are
reserved.  The behavior of upper bits being flags is firmly Intel-only.

Fixes: ca724305a2b0 ("KVM: x86/vPMU: Implement AMD vPMU code for KVM")
Tested-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Link: https://lore.kernel.org/r/20240109230250.424295-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>