perf/x86: Simplify x86_pmu.get_constraints() interface

perf/x86: Simplify x86_pmu.get_constraints() interface

There is a special case for validate_events() where we'll call
x86_pmu.get_constraints(.idx=-1). It's purpose, up until recent, seems
to be to avoid taking a previous constraint from
cpuc->event_constraint[] in intel_get_event_constraints().

(I could not find any other get_event_constraints() implementation
using @idx)

However, since that cpuc is freshly allocated, that array will in fact
be initialized with NULL pointers, achieving the very same effect.

Therefore remove this exception.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Stephane Eranian <eranian@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Ingo Molnar <mingo@kernel.org>