drm/amdgpu: modify a pair of functions for the pcie port wreg/rreg

This patch will modify a pair of functions for pcie port wreg/rreg.
AMD GPU have had an independent NBIO block from SOC15 arch.
If the dirver wants to read/write the address space of the pcie devices,
it has to go through the NBIO block.
This patch will move the pcie port wreg/rreg functions to
"amdgpu_device.c", so that to reuse the functions on the
future GPU ASICs.

Signed-off-by: Xiaojian Du <Xiaojian.Du@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>