sched: Check sched_domain before computing group power

After commit 863bffc80898 ("sched/fair: Fix group power_orig
computation"), we can dereference rq->sd before it is set.

Fix this by falling back to power_of() in this case and add a comment
explaining things.

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
[ Added comment and tweaked patch. ]
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: mikey@neuling.org
Link: http://lkml.kernel.org/r/20131113151718.GN21461@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>