drm/i915: Fix MST link rate handling

Now that intel_dp_max_link_bw() no longer considers the source
restrictions we may try to enable MST with 5.4GHz even when the source
doesn't support it. To fix that switch the code over to handle the link
rate in the same way as the SST code handles it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>