x86/pmu: Add pmu_caps flag to track if CPU is Intel (versus AMD)

Add a flag to track whether the PMU is backed by an Intel CPU.  Future
support for AMD will sadly need to constantly check whether the PMU is
Intel or AMD, and invoking is_intel() every time is rather expensive due
to it requiring CPUID (VM-Exit) and a string comparison.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221102225110.3023543-26-seanjc@google.com