drm/i915: Split intel_ddi_encoder_reset() from intel_dp_encoder_reset()

Most of intel_dp_encoder_reset() is for pre-ddi platforms.
Make a clean split.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210318161015.22070-5-ville.syrjala@linux.intel.com