drm/i915/mst: Force modeset on MST slaves when master needs a modeset

MST master can not be disabled while it have attached MST slaves, so
it is necessary force a modeset in all of its slaves.

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>
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/20191223010654.67037-6-jose.souza@intel.com