drm/amd/display: Remove OPP clock programming on plane disable

[Why]
Plane disable gets calls when we enable blank. On DCN2, we blank by using
DPG to display a black colour instead of using OTG blank. DPG runs off the
OPP clock, therefore we shouldn't disable the OPP clock when disabling the
plane. We do need to disable the OPP clock when disabling the entire pipe,
that will be addressed in a separate commit.

Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>