msm: mdss: turn off the DSI clocks on command mode panel

On command mode panel, panel is refreshing itself using
its own internal memory. Shutting down the dsi clocks
when there is no display update will save power.

Change-Id: Ibce33bed8c5ed65581557089b6537b1a8d9428b4
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>
Signed-off-by: Terence Hampson <thampson@codeaurora.org>