sched/fair: Fix group power_orig computation

When looking at the code I noticed we don't actually compute
sgp->power_orig correctly for groups, fix that.

Currently the only consumer of that value is fix_small_capacity()
which is only used on POWER7+ and that code excludes this case by
being limited to SD_SHARE_CPUPOWER which is only ever set on the SMT
domain which must be the lowest domain and this has singleton groups.

So nothing should be affected by this change.

Cc: Michael Neuling <mikey@neuling.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-db2pe0vxwunv37plc7onnugj@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>