drm/amdgpu: add a new helper to free a BO in kernel allocations

Free the BO allocated by amdgpu_bo_create_kernel()

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