drm/amd/display: fix trigger not generated for freesync

[Why]
In newer hardware MANUAL_FLOW_CONTROL is not a trigger bit. Due to this
front porch is fixed and in these hardware freesync does not work.

[How]
Change the programming to generate a pulse so that the event will be
triggered, front porch will be cut short and freesync will work.
Signed-off-by: Yogesh Mohan Marimuthu <yogesh.mohanmarimuthu@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn10/dcn10_optc.c

@@ -825,6 +825,9 @@ void optc1_program_manual_trigger(struct timing_generator *optc)
 
 	REG_SET(OTG_MANUAL_FLOW_CONTROL, 0,
 			MANUAL_FLOW_CONTROL, 1);
+
+	REG_SET(OTG_MANUAL_FLOW_CONTROL, 0,
+			MANUAL_FLOW_CONTROL, 0);
 }