amdgpu/soc15: make the pcie index/data registers constant.

These don't seem to change at runtime, and the initialisers
are constant data. This could be improved by not selecting
the apu/non-apu path on each pcie read/write access.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>