drm/i915: factor out ddi_get_encoder_port

In the next patch we'll need to get at both the encoder's intel_digital_port
object - which maybe NULL for a CRT - and it's port, so factor out this
functionality.

No functional change.

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>