drm/amdgpu: add new virtual display ID

In the case of pass-through, amdgpu.ko may be included into a image
with the hard code ID therefore loading driver with specified virtual
display ID will lose efficacy when the BDF of GPU modifies.So add the
new ID string "all" for it as same as vf case what does.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Yintian Tao <yttao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>