drm/amd/display: Change how we disable pipe split

Before this change, pipe split was disabled by bumping up dpp clock
bounding box for DPM level 0 and 1, this allows validation to pass
without splitting at a lower DPM level. This change reverts this
and instead lowers display clock at DPM level 0, this forces
configurations that need pipe split at DPM level 0 to go to
DPM level 1, where they can be driven without split.

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>