drm/radeon: fix power state when port pm is unavailable (v2)

When PCIe port PM is not enabled (system BIOS is pre-2015 or the
pcie_port_pm=off parameter is set), legacy ATPX PM should still be
marked as supported. Otherwise the GPU can fail to power on after
runtime suspend. This affected a Dell Inspiron 5548.

Ideally the BIOS date in the PCI core is lowered to 2013 (the first year
where hybrid graphics platforms using power resources was introduced),
but that seems more risky at this point and would not solve the
pcie_port_pm=off issue.

v2: agd: fix typo

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98505


Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: <stable@vger.kernel.org> # 4.8+
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>