drm/amdgpu: move pci handling out of pm ops

The documentation says the that PCI core handles this
for you unless you choose to implement it.  Just rely
on the PCI core to handle the pci specific bits.

Reviewed-by: Zhan Liu <zhan.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>