sched: remove rq->cpu_load[load_idx]

Remove 5 kinds of load_idx from rq->cpu_load, busy_idx, idle_idx,
newidle_idx, wake_idx, forkexed_idx.

Signed-off-by: Alex Shi <alex.shi@linaro.org>
sched: clean up source_load function

rq->cfs.runnable_load_avg is impossible great then rq->cpu_load. So
whenever LD_BIAS true or false, the function always return the mininal
value: rq->cfs.runnable_load_avg.

Signed-off-by: Alex Shi <alex.shi@linaro.org>