drm/i915/dp: abstract training pattern selection

drm/i915/dp: abstract training pattern selection

Make it cleaner to add more checks in the function. No functional
changes.

Cc: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Cc: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Cc: drm-intel-fixes@lists.freedesktop.org # dependency on the next patch
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1454667370-8001-1-git-send-email-jani.nikula@intel.com
(cherry picked from commit 23a5110dc619073b57d90c36eae383f51df03aac)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>