drm/i915/dp_mst: Enable VC payload allocation after transcoder is enabled

The spec requires enabling the MST Virtual Channel payload allocation
- in a separate step - after the transcoder is enabled, follow this.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200623082411.3889-1-imre.deak@intel.com