drm/amd/display: make underflow status clear explicit

[why]
HUBP underflow is never cleared, which causes underflow in one
test to fail another test, violating the independence requirements

[how]
Rather than make clearing implicit, we explicitly clear underflow
status in DTN.

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