drm/amd/amdgpu: Fix csb.bo pin_count leak on gfx 9

[Why]
csb bo is not unpinned in gfx 9. It will lead to pin_count leak on
driver unload.

[How]
Call bo_free_kernel corresponding to bo_create_kernel in
gfx_rlc_init_csb. This will also unify the code path with other gfx
versions.

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