drm/i915: Move register write into intel_dp_set_signal_levels()

Move register write from intel_dp_update_link_train() into
intel_dp_set_signal_levels(). This creates a better split between the
i915 specific code and the generic link training part. Note that this
causes an extra register write in intel_dp_reset_link_train(), since
both intel_dp_set_signal_levels() and intel_dp_set_link_train() write
to the DP register.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1445594525-7174-5-git-send-email-ander.conselvan.de.oliveira@intel.com