perf/x86/intel: Avoid rewriting DEBUGCTL with the same value for LBRs

perf with LBRs on has a tendency to rewrite the DEBUGCTL MSR with
the same value. Add a little optimization to skip the unnecessary
write.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: eranian@google.com
Link: http://lkml.kernel.org/r/1426871484-21285-2-git-send-email-andi@firstfloor.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>