drm/msm/mdp5: get the core clock rate from MDP5 config

The core clock rate depends on the hw configuration. Once we have
read the hardware revision, we can set the core clock to its
maximum value.
Before then, the clock is set at a rate supported by all MDP5
revisions.

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