video: fbdev: msm: store backlight level sent during handoff

Currently backlight level set before handoff is lost in blank
and unblank events. Store any unset bl_level during blank.

Change-Id: I5285ba5f6bea1d2045012057a209b95c7b5a9988
Signed-off-by: Venkata Prahlad Valluru <vvalluru@codeaurora.org>