drm/amdgpu: simplify runtime suspend

In the standard _PR3 case, the pci core handles the pci state.
The driver only needs to handle it in the legacy ATPX case.

This may fix issues with runtime suspend/resume on certain
hybrid graphics laptops.

Acked-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>