KVM: x86/pmu: Move initialization of valid PMCs bitmask to common x86

KVM: x86/pmu: Move initialization of valid PMCs bitmask to common x86

Move all initialization of all_valid_pmc_idx to common code, as the logic
is 100% common to Intel and AMD, and KVM heavily relies on Intel and AMD
having the same semantics.  E.g. the fact that AMD doesn't support fixed
counters doesn't allow KVM to use all_valid_pmc_idx[63:32] for other
purposes.

Tested-by: Xudong Hao <xudong.hao@intel.com>
Link: https://lore.kernel.org/r/20250806195706.1650976-31-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>