drm/i915/dp: Fix eDP on PCH DP on CPT/PPT

According to the gen6 docs, only the DP_A port (on-CPU eDP) still uses
the old IBX bit shift for the link training pattern setup bits.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>