drm/amd/display: clear optc underflow before turn off odm clock
[Why]
After ODM clock off, optc underflow bit will be kept there always and clear not work.
We need to clear that before clock off.
[How]
Clear that if have when clock off.
Reviewed-by: Alvin Lee <alvin.lee2@amd.com> Acked-by: Tom Chung <chiahsuan.chung@amd.com> Signed-off-by: Fudong Wang <Fudong.Wang@amd.com> Tested-by: Daniel Wheeler <daniel.wheeler@amd.com> Signed-off-by: Alex Deucher <alexander.deucher@amd.com>