drm/msm/mdp5: introduce mdp5_cfg module

The hardware configuration modification from a version to another
is quite consequent. Introducing a configuration module
(mdp5_cfg) may make things more clear and easier to access when a
new hardware version comes up.

Signed-off-by: Stephane Viau <sviau@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>