drm/i915: fix intel_prepare_ddi

At the moment intel_prepare_ddi buffer will iterate through both MST and
CRT encoders, which is incorrect. Neither of these encoder types have an
embedding intel_digital_port object, so for these encoder types we will
use random data when dereferencing the corresponding
intel_digital_port->port field.

Introduced in
commit b403745c
Author: Damien Lespiau <damien.lespiau@intel.com>
Date:   Mon Aug 4 22:01:33 2014 +0100

    drm/i915: Iterate through the initialized DDIs to prepare their buffers

v2:
- fix getting at the port for MST encoders too
- make sure that intel_prepare_ddi_buffers() gets called for port E too
  (Paulo)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90067


Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>