drm/i915/display: Prepare for fastset external dependencies check

drm/i915/display: Prepare for fastset external dependencies check

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