msm: mdss: add null check for back-light pointer

For esd recovery case add null pointer check for
panel structure and backlight pointer to avoid
null pointer dereferencing.

Change-Id: I79db326b95402a5c8d67d3880e4bb4b1a2c8aeae
Signed-off-by: Ramakant Singh <ramaka@codeaurora.org>