drm/i915/dp: Compute port sync crtc states post compute_config()

This patch pushes out the computation of master and slave
transcoders in crtc states after encoder's compute_config hook.
This ensures that the assigned master slave crtcs have exact same
mode and timings which is a requirement for Port sync mode
to be enabled.

v3:
* Make crtc_state const, remove crtc state NULL init (Ville)
v2:
* Correct indentation
* Rename to intel_ddi_port_sync_transcoders (Ville)
* remove unwanted debug (Ville)

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200214114126.13192-2-manasi.d.navare@intel.com