drm/amd/display: Add debugfs for forcing stream timing sync

[why]
There's currently no method to enable multi-stream synchronization from
userspace and we don't check the VSDB bits to know whether or not
specific displays should have the feature enable.

[how]
Add a debugfs entry that controls a new DM debug option,
"force_timing_sync". This debug option will set on any newly created
stream following the change to the debug option.
Expose a new interface from DC that performs the timing sync and a helper
to the "force_timing_sync" debugfs that iterates over the current streams
and modifies the current synchornization state and grouping.

Example usage to force a resync (from an X based desktop):

echo 1 > /sys/kernel/debug/dri/0/amdgpu_dm_force_timing_sync
xset dpms force off && xset dpms force on

Signed-off-by: Victor Lu <victorchengchi.lu@amd.com>
Reviewed-by: Aurabindo Jayamohanan Pillai <Aurabindo.Pillai@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Acked-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h

@@ -340,6 +340,7 @@ struct amdgpu_display_manager {
 	 * fake encoders used for DP MST.
 	 */
 	struct amdgpu_encoder mst_encoders[AMDGPU_DM_MAX_CRTC];
+        bool force_timing_sync;
 };
 
 struct dsc_preferred_settings {
@@ -493,6 +494,8 @@ void dm_restore_drm_connector_state(struct drm_device *dev,
 void amdgpu_dm_update_freesync_caps(struct drm_connector *connector,
 					struct edid *edid);
 
+void amdgpu_dm_trigger_timing_sync(struct drm_device *dev);
+
 #define MAX_COLOR_LUT_ENTRIES 4096
 /* Legacy gamm LUT users such as X doesn't like large LUT sizes */
 #define MAX_COLOR_LEGACY_LUT_ENTRIES 256