drm/amdgpu/atom: fix atom_fw check

Not all vbios images seem to set the version appropriately.
Switch the check based on asic type instead.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>