drm/i915/tgl: do not enable transcoder clock twice on MST

For MST on Tiger Lake there are different moments when we need to
configure the transcoder clock select. For the first link this is in step
7.a of the spec, before training the link.  For additional streams this
should be done as part of step 8.b after programming receiver VC Payload
ID.

Bspec: 49190

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191030012448.14937-4-lucas.demarchi@intel.com