drm/i915/display: Remove port and phy from voltage swing functions

This information can be get directly from intel_encoder so no need
of those parameters.

Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200708205512.21625-2-jose.souza@intel.com