drm/i915: Initialize panel_vdd_work only for eDP ports

It is only used by eDP ports so no need to initialize it for each DP
port.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181030215750.28213-4-jose.souza@intel.com