drm/i915/tgl: add support to one DP-MST stream

This is the minimum change to support 1 (and only 1) DP-MST monitor
connected on Tiger Lake. This change was isolated from previous patch
from José. In order to support more streams we will need to create a
master-slave relation on the transcoders and that is not currently
working yet.

v2: remove unused macro and use REG_FIELD_PREP() (Ville)

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/20191029035049.5907-1-lucas.demarchi@intel.com