drm/i915: Update dbuf slices only with full modeset

During full modeset, global state(i.e dev_priv) is protected
by locking the crtcs in state, otherwise global state is not
serialized. Also if it is not a full modeset, we anyway
don't need to change DBuf slice configuration as Pipe configuration
doesn't change.

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/20200202230630.8975-4-stanislav.lisovskiy@intel.com