drm/i915: check eDP encoder correctly when setting modes

We were using a stale pointer in the check which caused us to use CPU
attached DP params when we should have been using PCH attached params.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=31988
Tested-by: Jan-Hendrik Zab <jan@jhz.name>
Tested-by: Christoph Lukas <christoph.lukas@gmx.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org