sched: ems: take current capacity into account when choosing prefer_idle CPUs

We want the CPUs to stay at the lowest OPP as possible, and avoid excessive
task packing, as prefer_idle tasks are latency-sensitive tasks

Signed-off-by: Diep Quynh <remilia.1505@gmail.com>
Change-Id: Id0ef06c1dd1e67a36a94cdffaba0d9dcc9c89f62