drm/i915: Extend DSL readout fix to BDW and SKL.

drm/i915: Extend DSL readout fix to BDW and SKL.

Those platforms have the same bug as haswell, and the same fix applies
to them.

The original HSW fix that this extends is

commit 26b8d443b460baa6b750f9d8f0cb31155bd36cc3
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Tue Sep 22 12:15:54 2015 -0700

    drm/i915: workaround bad DSL readout v3

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: stable@vger.kernel.org # v4.3
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91579
Link: http://patchwork.freedesktop.org/patch/msgid/1446535913-31970-3-git-send-email-maarten.lankhorst@linux.intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>