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>