drm/amdgpu: make userq fence_drv drop explicit in queue destroy

drm/amdgpu: make userq fence_drv drop explicit in queue destroy

amdgpu_userq_fence_driver_free() is now responsible only for releasing
per-queue ancillary state (last_fence, fence_drv_xa) and no longer
touches the ownership reference, making each function's contract clear.

v2: Get the userq fence driver from amdgpu_userq_fence_driver_alloc()
    directly and dropping the userq fence driver reference after removing
    userq_doorbell_xa entry.(Christian)

Signed-off-by: Prike Liang <Prike.Liang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>