drm/amdgpu: add initial support for sdma v6.0

Add functions for SDMA version 6.

Signed-off-by: Stanley Yang <Stanley.Yang@amd.com>
Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>