[Why]
Display Manager initializes array of vblank workqueues, but only 1 is used.

[How]
Use single instance init instead of array.
Reviewed-by: NQingqing Zhou <Qingqing.Zhuo@amd.com>
Acked-by: NAurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: NRoman Li <Roman.Li@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>