drm/amdgpu: store userq_managers in a list in adev

So we can iterate across them when we need to manage
all user queues.

v2: add uq_mgr to adev list in amdgpu_userq_mgr_init

Reviewed-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>