drm/radeon/kms: fix hdmi duallink checks

All pre-SI chips are limited to 165 Mhz for single link.
Code in question will be re-enabled when SI support is added.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=44755
https://bugzilla.kernel.org/show_bug.cgi?id=42887

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>