drm/amdgpu: init gfx_v9_4_3 external_rev_id

it is used for user space driver to identify gfx_v9_4_3 chip

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