msm_fb: Set backlight to zero during adb shell stop scenario

During framework reboot (adb shell stop/start), the backlight
value might be non-zero after adb shell stop. In such cases,
during adb shell start, we see momentary garbled screen due to
panel-init pattern since backlight is ON. Hence, set backlight
to zero during adb shell stop. Also, store the latest backlight
value before adb shell stop so that backlight can be set soon
after first frame update after adb shell start.

CRs-fixed: 479316
Change-Id: I2a98c2599f5f3fadf516da2df06556f52fe50a2e
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>