drm/i915: Store port enum in intel_encoder

Storing the port enum in intel_encoder makes it convenient to know the
port attached to an encoder. Moving the port information up from
intel_digital_port to intel_encoder avoids unecessary intel_digital_port
access and handles MST encoders cleanly without requiring conditional
checks for them (thanks danvet).

v2:
Renamed the port enum member from 'attached_port' to 'port' (danvet)
Fixed missing initialization of port in intel_sdvo.c (danvet)

v3:
Fixed missing initialization of port in intel_crt.c (Ville)

v4:
Storing port for DVO encoders too.

Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Lyude <cpaul@redhat.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474334681-22690-3-git-send-email-dhinakaran.pandiyan@intel.com