msm: mdss: turn on the dsi clocks before sending dsi commands

The dsi clocks can be turned off after a period of screen
idle time.  Before sending the dsi commands, we need to
make sure the dsi clocks are on.

Change-Id: Idc5c803aa6eb55d1739441d1bd395fed5c2d74c7
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>