drm/i915/display: Use external dependency loop for port sync

This loop was added directly to intel_atomic_check() to be used by
all other features that have external pipe dependencies, so using it
and removing intel_atomic_check_synced_crtcs().

After this changes is_trans_port_sync_master() it not used anywhere,
so removing it.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200106142823.145260-1-jose.souza@intel.com