drm/amdgpu: use amdgpu_bo_[create|free]_kernel for wb

Rather than open coding it.

Acked-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>