drm/amd: make a type-safe cgs_device struct. (v2)

This is just a type-safety things to avoid everyone taking void *,
it doesn't change anything.

v2: agd5f: split out the dal changes into a separate patch.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>