drm/amdgpu: use pcie functions for link width and speed

Use the newly exported pci functions to get the link width
and speed rather than using the drm duplicated versions.

Also query the GPU link caps directly rather than hardcoding
them.

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