drm/amdgpu: fallback to generic HDP operation

When ring special operations aren't available we can fallback to the
generic ASIC operations.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>