msm: mdss: validate fb backlight inputs

Ensure backlight level is less than the max backlight
brightness and the backlight scaling factor is at most
1.

Change-Id: I424ed22af48ecae46371291fe88a114cf3fc7c31
CRs-Fixed: 516699
Signed-off-by: Manoj Rao <manojraj@codeaurora.org>