drm/amdgpu: Remove duplicate code in gfx_v8_0.c

There are no any logical changes here.

1. if kcq can be enabled via kiq, we don't need to
   do kiq ring test.
2. amdgpu_ring_test_ring function can be used to
   sync the ring complete, remove the duplicate code.

v2: alloc 6 (not 7) dws for unmap_queues

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