drm/i915: Replicate BIOS eDP bpp clamping hack for hsw

Haswell's DDI encoders have their own ->get_config callback and in

commit c6cd2ee2
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Mon Oct 21 10:52:07 2013 +0300

    drm/i915/dp: workaround BIOS eDP bpp clamping issue

we've forgotten to replicate this hack. So let's do it that.

Note for backporters: The above commit and all it's depencies need to
be backported first.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=71049


Cc: stable@vger.kernel.org
Tested-by: Gökçen Eraslan <gokcen.eraslan@gmail.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>