drm/i915: Revert DisplayPort fast link training feature

drm/i915: Revert DisplayPort fast link training feature

It has been found out that in some HW combination the DisplayPort
fast link training feature caused screen flickering. Let's revert
this feature for now until we can ensure that the feature works for
all platforms.

This is a manual revert of commits 69f9308f2555 ("drm/i915: DP link
training optimization") and d6f457f5c7dd ("drm/i915: eDP link training
optimization").

Fixes: 69f9308f2555 ("drm/i915: DP link training optimization")
Fixes: d6f457f5c7dd ("drm/i915: eDP link training optimization")
Cc: <stable@vger.kernel.org> # v4.2+
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91393
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466410226-19543-1-git-send-email-mika.kahola@intel.com
(cherry picked from commit 91df09d92ad82c8778ca218097bf827f154292ca)