drm/msm/mdp: Remove CONFIG_OF checks from MDP drivers

We don't intend to use downstream non-DT kernels anymore, so remove
CONFIG_OF checks.

Update the TODO comment so that we don't forget about max_clk setting
for non APQ8064 chips having MDP4.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>