drm/i915: use vlv_dport_to_channel in vlv_signal_levels

Minor cleanup.  Would be nice to use an enum for channel in the DPIO
macros so we don't mix up pipes and channels, but that's for another
patch.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>