drm/i915: Introduce encoder->compute_config_late()

Add an optional secondary encoder state compute hook. This gets
called after the normak .compute_config() has been called for
all the encoders in the state. Thus in the new hook we can rely
on all derived state populated by .compute_config() to be already
set up. Should be useful for MST and port sync master/slave
transcoder selection.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200214114126.13192-1-manasi.d.navare@intel.com