drm/amdgpu: remove unnecessary logic of ASIC check

Remove some unused ASIC check logic.
Remove some definition of amdgpu_device which only used by
the removed ASIC check logic.(V2)

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>