drm/i915/mst: Use .compute_config_late() to compute master transcoder

drm/i915/mst: Use .compute_config_late() to compute master transcoder

Use the recently introduced encoder .compute_config_late() hook to
do the MST master transcoder assignment. Avoids having to do it
in a funny way before we know the CPU transcoder of each pipe.

And now we can also properly use hw.active instead of uapi.active
since it too has been calculated earlier for everyone.

Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200313164831.5980-2-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>