drm/amdgpu: Fix pcie_bw on Vega20

The registers used for VG20 are different in that certain performance
counters were split off to TXCLK3/4. Vega10/12 doesn't have this, so add
a new vg20_get_pcie_usage to reflect this change.

Signed-off-by: Kent Russell <kent.russell@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>