drm/i915: Move dbuf slice update to proper place

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: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200202230630.8975-3-stanislav.lisovskiy@intel.com