drm/amdgpu: cache gpu pcie link width

Get the PCIe link with of the device itself (or it's
integrated upstream bridge) and cache that.

v2: fix typo

Link: drm/amd#3820


Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>