msm: pm-8x60: Add early return to msm_pm_enable_retention()

If retention is already disabled, it is not necessary to send IPIs to
the running CPUs to wake them up. Insert an early return for this
scenario to reduce overhead during Krait rail voltage scaling, which
calls this API on every voltage change.

Change-Id: I5db0bafae187ff8a68488e86d4787df9c45787e4
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>