drm/amd/display: Remove redundant vblank workqueues in DM

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

[How]
Use single instance init instead of array.

Reviewed-by: Qingqing Zhou <Qingqing.Zhuo@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Roman Li <Roman.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>