Current DBuf slices update wasn't done in proper
place, especially its "post" part, which should
disable those only once vblank had passed and
all other changes are committed.

v2: Fix to use dev_priv and intel_atomic_state
    instead of skl_ddb_values
    (to be nuked in Villes patch)

v3: Renamed "enabled_slices" to "enabled_dbuf_slices_num"
    (Matt Roper)

v4: - Rebase against drm-tip.
    - Move post_update closer to optimize_watermarks,
      to prevent unneeded noise from underrun reporting
      (Ville Syrjälä)
Reviewed-by: NMatt Roper <matthew.d.roper@intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NStanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200202230630.8975-3-stanislav.lisovskiy@intel.com