drm/amdgpu: Refine gfx_v8_0_kcq_disable function

Send all kcq unmap_queue packets and then wait for
complete.

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>