Use amdgpu_bo_free_kernel instead of amdgpu_bo_unref to
perform a proper cleanup of PDB bo.

v2: update subject to be more accurate
Signed-off-by: NGuchun Chen <guchun.chen@amd.com>
Reviewed-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>