drm/i915/display: Fix TRANS_DDI_MST_TRANSPORT_SELECT definition

TRANS_DDI_MST_TRANSPORT_SELECT is 2 bits wide not 3, it was taking
one bit from EDP/DSI Input Select.

Fixes: b3545e08

 ("drm/i915/tgl: add support to one DP-MST stream")
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107214559.77087-1-jose.souza@intel.com
(cherry picked from commit bb747fa5

)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>