msm: mdss: add additional blending stages for the layer mixers

msm8994 can support three additional blending stages in the layer
mixer bringing the total to 8 layers including the base stage.
Modify layer mixer programming to support additional blend stages.

Change-Id: Iff812ec2cf08e4234dc13c5630fe66ba07462b87
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>