drm/amd/display: trigger timing sync only if TG is running

[Why&How]
If the timing generator isnt running, it does not make sense to trigger
a sync on the corresponding OTG. Check this condition before starting.
Otherwise, this will cause error like:

*ERROR* GSL: Timeout on reset trigger!

Fixes: dc55b106 ("drm/amd/display: Disable phantom OTG after enable for plane disable")
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>