cpuidle: lpm-levels: update lpm_disallowed() with sched hints

lpm_disallowed() should consider hint from scheduler via
sched_lpm_disallowed_time(), update it accordingly.
While at it, clean-up functions and move them under WALT config.

Change-Id: I7bb38951c4a892b86e50458a5c10e5e7497e9b84
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>