drm/msm/dsi-staging: send backlight commands in dsi active state.

Add a check to make sure that dsi panel is in
initialized state to send backlight commands to
the panel.

Change-Id: Ic5ea72b1dc86e6b1968625f55789979ffbe1980f
Signed-off-by: Vara Reddy <varar@codeaurora.org>