drm/i915/tgl: Add missing ddi clock select during DP init sequence

Step 4.b was complete missed because it is only required to TC and TBT.

Bspec: 49190
Reviewed-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Clinton A Taylor <clinton.a.taylor@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190920205810.211048-2-jose.souza@intel.com