msm: mdss: Change the Alpha scaling type to bilinear

Currently, for Alpha plane the filter used is nearest due to which
there is an aliasing effect. Using the same filter as graphics core
which is bilinear, the aliasing effect is avoided.

CRs-fixed: 517627
Change-Id: Ifb4629ddcea1ac447d151469b1f56e3400b7fac0
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>