drm/i915: switch intel_ddi_init() to intel types

Prefer using intel_encoder and pass the base where needed rather than
keeping both encoder and intel_encoder variables around.

v2: actually add all changes to the patch

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191106071715.10613-1-lucas.demarchi@intel.com