msm: pm2: Use a different kernel API to get the sleep length

Currently pm2 driver calls the "msm_timer_enter_idle" &
msm_timer_exit_idle" API's for all the power modes (i.e.
SWFI, GDFS/StandalonePC) to get next timer expiration &
time sync with modem respectively. Use these API's only
when the system decide to enter into a modem assisted PC
otherwise use a differnt API to get the sleep length.

Change-Id: Idf5d53518f75551bab432b3532c0eae1f61cdff1
Signed-off-by: Murali Nalajala <mnalajal@codeaurora.org>