drm/amd/display: remove stale debug setting

[Why&How]
The debug option to disable idle power optimization can be dropped

Fixes: 235c6763 ("drm/amd/display: add DCN32/321 specific files for Display Core")
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>