drm/amdgpu: add some basic elements for multiple XCD case

Add some basic definitions and structure member. Inscrease MAX_WB slots
to 1024 to support the increasing number of rings for multiple partitions.

v2: unify naming style

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>