drm/amd/display: clear underflow on optc unblank

[why]
Underflow is asserted due to some timing condition which does not
actually result in visible underflow (i.e. it occurs while blanked).

[how]
Force clear underflow occured bit whenver we unblank.

Signed-off-by: Jun Lei <Jun.Lei@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>