drm/i915: Split write of pattern to DP reg from intel_dp_set_link_train

Split the register write with the new link training pattern out of
intel_dp_set_link_train(), so that the i915 specific code is in a
separate function.

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/1445594525-7174-3-git-send-email-ander.conselvan.de.oliveira@intel.com