msm: mdss: Correct the AD backlight calculation formulas

The previous AD backlight calculation formulas assume that the
BL_LIN, BL_LIN_INV and BL_ATT LUTs are monotonically increasing,
which can cause miscalculation when non monotonic LUTs are used.
This patch fixes the problem by removing the above assumption.

Change-Id: Ifee9dc067de8c6dbd24aac407ba702a13241f782
Signed-off-by: Ping Li <pingli@codeaurora.org>