drm/msm/dsi-staging: fix backlight update and dsi off race condition

In the current implementation there can be a scenario where in one
thread dsi shutdown sequence is taking place and at the same time
in another thread backlight update request might have come. This may
result dsi in a bad state when dsi off sequence is happening in one
thread and in the other thread backlight update will try to enable
dsi pll. So fix the race condition between dsi off and dcs backlight
update by making them mutually exclusive.

Change-Id: I8cecb007117893091aacc7ebe3c93cc0e2c59b13
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>
Signed-off-by: Lakshmi Narayana Kalavala <lkalaval@codeaurora.org>