msm_fb: display: turn mdp clock on at pipe_commit

mdp clock is controlled by frame work. There has
possibility that mdp clock had been turned off while
frame update still happen. In this case update
thread may stuck at wait4vsync since there has no
mdp clcok to trigger vsync interrupt. This patch
will enable mdp clock at pipe commit when mdp clock
is tunred off so that frame update can be completed.

Change-Id: I2d31b9a4b6f0cb9dd36516a80931988e23e8a545
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>