msm: mdss: ensure clocks are on till pixel transfer is completed

When pixel transfer is going on, there is a race condition during
which pixel clock can get turned off due to early clock gating
before CMD_MDP interrupt is triggered which can cause interrupt
storm. This change ensures CMD_MDP interrupt is received before
turning off clocks.

Change-Id: I33b664334912584170c12733329051b067c3f1da
Signed-off-by: Ashish Garg <ashigarg@codeaurora.org>