drm/amdgpu/gfx10: fix unbalanced MAP/UNMAP_QUEUES when async_gfx_ring is disabled

gfx_v10_0_kiq_enable_kgq() is called only when async_gfx_ring is
enabled, so should gfx_v10_0_kiq_disable_kgq().

Signed-off-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jack Xiao <Jack.Xiao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>