drm/amdgpu: Don't enable LTR if not supported

As per PCIE Base Spec r4.0 Section 6.18
'Software must not enable LTR in an Endpoint unless the Root Complex
and all intermediate Switches indicate support for LTR.'

This fixes the Unsupported Request error reported through AER during
ASPM enablement.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=216455

The error was unnoticed before and got visible because of the commit
referenced below. This doesn't fix anything in the commit below, rather
fixes the issue in amdgpu exposed by the commit. The reference is only
to associate this commit with below one so that both go together.

Fixes: 8795e182

 ("PCI/portdrv: Don't disable AER reporting in get_port_device_capability()")

Reported-by: Gustaw Smolarczyk <wielkiegie@gmail.com>
Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>

Cc: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>