drm/radeon: allow pcie gen2 speed on NI

Enabling pcie gen2 speed was skipped for Northern Islands
AISCs, although it looks like it works just fine with the same
initialization sequence used for evergreen.

According to Alex D. gen2 init was skipped to prevent a crash
that has been caused by some other bug that has been
fixed in the meantime; so now it should be safe to enable it.

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>