drm/amdgpu: Adjust logic in amdgpu_device_partner_bandwidth()

Use current speed/width on devices which don't support
dynamic PCIe switching.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3289


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