drm/amdgpu: fix amdgpu pmu to use hwc->config instead of hwc->conf

hwc->conf was designated specifically for AMD APU IOMMU purposes.  This
could cause problems in performance and/or function since APU IOMMU
implementation is elsewhere.  Also hwc->conf and hwc->config are
different members of an anonymous union so hwc->conf aliases as
hw->last_tag.

Signed-off-by: Jonathan Kim <Jonathan.Kim@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>