drm/i915: Extract intel_disable_ddi_buf()

Extract the code to disable the DDI_BUF_CTL into small helper. This
will allows us to detangle the encoder type mess in
intel_ddi_post_disable().

v2: Keep using intel_ddi_get_encoder_port() for now

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171010121207.570-4-ville.syrjala@linux.intel.com