drm/i915: Remove useless eDP check from intel_ddi_pre_enable_dp()

intel_edp_panel_on() will itself do the is_edp() check, so the caller
doesn't have to bother. Pre-DDI code doesn't bother, so let's follow the
same approach for DDI.

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/20171010121207.570-7-ville.syrjala@linux.intel.com