msm: mdss: refactor mixer configuration code

Current mixer configuration code is scattered across multiple places,
with the addition of extension registers and additional mixers, there
are some changes that are not properly handled in all places leading
to issues in configuration that are hard to debug.

Centralize the programming of mixer configuration into a few functions
that handle overall programming, making it easier to maintain.

CRs-Fixed: 987777
Change-Id: I76410ea8eb1ea42ad42613fdb844ce6a8eb8c3ff
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>