mdss: mdp3: Remove extra call to clk_prepare

clk_prepare got moved inside mdp3_clk_update. Prior to this patch
unprepare would never be called successfully.

Change-Id: I28ab035b42d433f379d63748ac4f506a8575ea66
Signed-off-by: Terence Hampson <thampson@codeaurora.org>