MST and port sync have master and slaves pipes and it brings
dependencies between pipes to allow fastset.
For example if only MST master needs a modeset all of its slaves also
needs to do a modeset.

This patch adds the base for external dependencies check, the MST and
port sync bits will be added in another patches.

v3:
- moved handling to intel_atomic_check() this way is guarantee that
all pipes will have its state computed

v4:
- added a function to return if MST master neeeds modeset to simply
code in intel_atomic_check()

v5:
- fixed and moved code to check if MST master needs a modeset

v6:
- previons version of this patch was split into two patches

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191223010654.67037-5-jose.souza@intel.com