drm/i915: Standardize port type for DVO encoders

Changing the return type from 'char' to 'enum port' in
intel_dvo_port_name() makes it easier to later move the port information to
intel_encoder. In addition, the port type conforms to what we have
elsewhere.

Removing the last conditional that handles invalid port because dvo_reg is
intialized to valid values for all DVO devices at definition.

v2: Changed return type, for real (Jani)

Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Lyude <cpaul@redhat.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474334681-22690-2-git-send-email-dhinakaran.pandiyan@intel.com