msm: pm-8x60: Merge msm_pm_idle_prepare into msm_pm_idle_enter

Modify LPM drivers to call msm_pm_idle_enter when idle. enter_sleep will
look at available time to sleep and the latencies of different low power
modes and decide the low power mode to enter. It returns the low power
mode enumeration back to cpuidle.

Change-Id: I56717f2ba7a748b38d53b418381a01159055e7bd
Signed-off-by: Priyanka Mathur <pmathur@codeaurora.org>
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>

Conflicts:
	arch/arm/mach-msm/no-pm.c
	arch/arm/mach-msm/pm-8x60.c