fbdev: msm: call pxl clk unprepare during suspend

Pxl clock needs to be un-prepared during device goes to suspend.
This will avoid byte clk warning back-trace in the logs and failure
to enable the clocks.

Change-Id: Iab6649af34d0135475e05d86f6d8545d20869abb
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>