drm/amdgpu:Fix the unpin warning about csb buffer

As it will destroy clear_state_obj, and also will unpin it in the
gfx_v9_0_sw_fini, so don't need to
call amdgpu_bo_free_kernel in gfx_v9_0_sw_fini, or it will have unpin warning.

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