drm/amd/display: Clear GPINT1 before taking DMCUB out of reset

[Why]
Workaround for DMCUB front door load

[How]
Clear GPINT after reset so its consistent

Signed-off-by: Samson Tam <Samson.Tam@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>