msm: mdss: Turn off backlight while blank-unblank in continuous splash

Turn off the backlight while calling blank-unblank during continuous
splash as it could show white flicker depending on the panel.

CRs-Fixed: 532273
Change-Id: I99e9bff26b540f2e33bfeb13cbb29548feea599c
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>