sched: add check for cpu idleness when using C-state information

Task enqueue on a CPU occurs prior to that CPU exiting an idle state.
For the time duration between enqueue and idle exit, the CPU C-state
information can no longer be relied on for further task placement
since already enqueued/waiting tasks are not taken into account. The
small task placement algorithm implicitly assumes a non zero C-state
implies an idle CPU. Since this assumption is incorrect for the
duration described above, make the cpu_idle() check explicit. This
problem can lead to task packing beyond the mostly_idle threshold.

Change-Id: Idb5be85705d6b15f187d011ea2196e1bfe31dbf2
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>