FIR and M/N scalar when used for downscaling, drops alpha channel of the given layer. To notify this, alpha_drop flag is updated accordingly. This flag need not be checked for layers with no alpha plane. In some scenarios alpha_drop may get enabled for the layers with no alpha plane and cause issues realted to blend composition of layers. Check for setting alpha_drop only if source ayer has alpha plane. Change-Id: Ie64b30c045a78d0d68eb9efacc76902afae7850d Signed-off-by: Mayank Chopra <makchopra@codeaurora.org>