msm: mdss: dsi: fix post clk off callback in clk manager

DSI clock manager is registering the post clk on twice
but does not register the post clk off. CORE/CTRL/PHY
power modules were never turned off because post clk
off callback is not registered.

Change-Id: I324e416241d9ec72e0aef8c4742849c14cffc37e
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>