drm/msm/mdp5: Don't get source of MDP core clock

The driver expects DT to provide the parent to MDP core clock. The only
operation done to the parent clock is to set a rate. This can be
achieved by setting the rate on the core clock itsef. Don't try to
get the parent clock anymore.

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