drm/amd/display: Clear optc false state when disable otg

[why]
when disable optc, need to clear the underflow status as well.

Reviewed-by: Chris Park <Chris.Park@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>