msm: kgsl: unprepare clocks when we move from NAP to SLEEP

The existing code leaves clks prepared when we transition from
NAP to SLEEP. We fail to reach Vdd_min as voltage scaling has
moved to prepare/unprepare.

Change-Id: I509f4c6bd217ead985890147819236dd2c5d03ce
Signed-off-by: Suman Tatiraju <sumant@codeaurora.org>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>