Revert "drm/amd/display: change the panel power savings level without a modeset"

This reverts commit 8d20a066.

It's causing a failure in the abm_gradual igt test.

Signed-off-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>