KVM: arm64: Tighten the definition of FEAT_PMUv3p9

KVM: arm64: Tighten the definition of FEAT_PMUv3p9

The current definition of FEAT_PMUv3p9 doesn't check for the lack
of an IMPDEF PMU, which is encoded as 0b1111, but considered unsigned.

Use the recently introduced helper to address the issue (which is
harmless, as KVM never advertises an IMPDEF PMU).

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250714115503.3334242-6-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>