drm/msm/mdp5: Remove unused downstream bus scaling apis

MSM bus scaling has moved on to use interconnect framework
and downstream bus scaling apis are not present anymore.
Remove them as they are nop anyways in the current code,
no functional change.

Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>