drm/i915: apply DP bandwidth workaround for PCH eDP as well

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=29141

 though the
workaround itself is still a bit of a mystery.

Tested-by: Adam Hill <sidepipeuk@yahoo.co.uk>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>