drm/amdgpu: conduct a proper cleanup of PDB bo

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: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>