x86, sched: Account for CPUs with less than 4 cores in freq. invariance

If a CPU has less than 4 physical cores, MSR_TURBO_RATIO_LIMIT will
rightfully report that the 4C turbo ratio is zero. In such cases, use the
1C turbo ratio instead for frequency invariance calculations.

Fixes: 1567c3e3

 ("x86, sched: Add support for frequency invariance")
Reported-by: Like Xu <like.xu@linux.intel.com>
Reported-by: Neil Rickert <nwr10cst-oslnx@yahoo.com>
Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Link: https://lkml.kernel.org/r/20200416054745.740-3-ggherdovich@suse.cz