drm/msm/dsi-staging: disallow backlight update during panel mode switch

DSI controller and clock will be disabled/enabled during panel mode
switch, so disallow backlight update during panel mode switch to
avoiding DSI exception.

Change-Id: I37e2f3c9aa929555593ffb53950521150ee7698f
Signed-off-by: Lei Chen <chenlei@codeaurora.org>
Signed-off-by: Lipsa Rout <lrout@codeaurora.org>