msm_fb: Fix display corruption with continuous splash screen feature

During boot up, we observe random lines on screen between the
splash_screen image and the android animation. MDP clocks
were turned off during bootup. Fix this issue by enabling
the clocks when this feature is enabled.

Add check to see that the video mode engine is not busy before
turning off the dsi controller to avoid any underruns.

CRs-Fixed: 399634

Change-Id: I8fdb50fa848362a548b8cdd53ee1817346b57716
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>