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 5fa836a9 ("drm/i915: DP link
training optimization") and 4e96c977 ("drm/i915: eDP link training
optimization").

Fixes: 5fa836a9 ("drm/i915: DP link training optimization")
Fixes: 4e96c977 ("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 91df09d9)