msm: mdss: fix the mdp dsi clock dependency issue on 8x10

MDP DSI clock is coming from the DSI PLL.  During the shut
down process, the DSI PLL was powered off first before turning
off the MDP DSI clock.   This results in the mdp_dsi_clk stuck
on message.   The shutdown sequence is changed to: turn off
the panel, turn off the MDP clocks, turn off the DMA engine,
and turn off the dsi controller, including the DSI clocks.

Change-Id: Ib9b47c693188784e7f59d209d868b665ff0beef8
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>