drm/amdgpu: Fix pci platform speed and width

The new Vega series GPU cards have in-built bridges. To get the pcie
speed and width supported by the platform walk the hierarchy and get the
slowest link.

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