drm/msm/dpu: clean up references of DPU custom bus scaling

Since the upstream interconnect bus framework has landed
upstream, the existing references of custom bus scaling
needs to be cleaned up.

Changes in v2:
	- Fixed build error due to partial clean up

Changes in v3:
	- Condense multiple lines into a single line (Sean Paul)

Changes in v4-v7:
	- None

Signed-off-by: Sravanthi Kollukuduru <skolluku@codeaurora.org>
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Sean Paul <sean@poorly.run>