Add a global state to track the dbuf slices. Gets rid of all the nasty
coupling between state->modeset and dbuf recomputation. Also we can now
totally nuke state->active_pipe_changes.

dev_priv->wm.distrust_bios_wm still remains, but that too will get
nuked soon.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200225171125.28885-9-ville.syrjala@linux.intel.comReviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>