drm/amdgpu: fix the PCIe lanes reporting in the INFO IOCTL

Combine the platform and GPU caps like we do for PCIe Gen.
This aligns properly with expectations and documentation
for the interface.

Link: drm/amd#3820


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