msm: mdss: control backlight level during blank and unblank events

Add code to set the backlight level to zero for blank event
and restore the backlight level to its previous value during
unblank event. This will fix any unexpected panel flicker
that might show up on the screen during panel ON sequence.

CRs-Fixed: 608091
Change-Id: I871a067b0cc40d893b8b7e5a25103ebb914f9b7a
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>
Signed-off-by: Benet Clark <benetc@codeaurora.org>