msm: lpm_levels: Perform system_unprepare only system low power mode

system_unprepare() needs to execute operations related to system
unprepare only when the CPU subsystem was scheduled for a system low
power mode by the last core that went down. On the last core down, its
possible that residency conditions for system low power mode weren't met
and the system was never programmed to enter a low power mode. In these
scenarios, the system_unprepare() needn't execute any of the unprepare
functionalities.

CRs-fixed: 596831
Change-Id: I36b00f254354920775ba8e797bb08932e2495b24
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>