perf/x86: Rename x86_pmu::er_flags to 'flags'

Because it will be used for more than just tracking the
presence of extra registers.

Signed-off-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: bp@alien8.de
Cc: jolsa@redhat.com
Cc: kan.liang@intel.com
Cc: maria.n.dimakopoulou@gmail.com
Link: http://lkml.kernel.org/r/1416251225-17721-2-git-send-email-eranian@google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>