drm/i915: Drop pointless WARN_ON

intel_dp_link_down() is static and it's only called from the pre-ddi
DP functions, so having a WARN_ON(HAS_DDI) in there is quite pointless.
Remove it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190604140214.9947-2-ville.syrjala@linux.intel.com


Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>