drm/i915/display: Replace drm_i915_private in voltage swing functions by intel_encoder

intel_encoder will be needed inside of vswing functions in a future
patch, so here doing this change in all vswing functions since HSW.

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/20200708205512.21625-1-jose.souza@intel.com