drm/i915: add intel_dp_set_signal_levels

So we can de-duplicate code that's inside intel_dp_start_link_train
and intel_dp_complete_link_train.

V2: Rebase since patch 3/5 was discarded.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>