drm/i915: Don't set update_wm_post on g4x+

update_wm_post is meant for pre-g4x only. Don't ever set
it on g4x+.

The only effect of a bogus update_wm_post on g4x+ could
be that we clear the legacy_cursor_update flag in
intel_atomic_commit(). Since legacy_cursor_update is
only set for legacy cursor updates (as the name suggests)
and we only set update_wm_post for a modeset the two
cases should never occur at the same time. But let's
be consistent in setting update_wm_post so we don't
end up confusing so many people.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190206185433.8116-1-ville.syrjala@linux.intel.comReviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -11063,7 +11063,8 @@ static int intel_crtc_atomic_check(struct drm_crtc *crtc,
 	int ret;
 	bool mode_changed = needs_modeset(crtc_state);
 
-	if (mode_changed && !crtc_state->active)
+	if (INTEL_GEN(dev_priv) < 5 && !IS_G4X(dev_priv) &&
+	    mode_changed && !crtc_state->active)
 		pipe_config->update_wm_post = true;
 
 	if (mode_changed && crtc_state->enable &&