drm/i915: Make intel_crtc_arm_fifo_underrun() functional on gen2

Assuming intel_crtc_arm_fifo_underrun() only gets called when
there's no pending plane updates we can utilize it on gen2 by
checking the active_planes bitmask so that we only re-enable
underrun reporting if some planes are active.
i915_fifo_underrun_reset_write() seems to have the necessary
hw_done/flip_done waits in place.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127190556.1574-8-ville.syrjala@linux.intel.comReviewed-by: José Roberto de Souza <jose.souza@intel.com>

drivers/gpu/drm/i915/display/intel_display.c

@@ -14221,7 +14221,7 @@ void intel_crtc_arm_fifo_underrun(struct intel_crtc *crtc,
 {
 	struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);
 
-	if (!IS_GEN(dev_priv, 2))
+	if (!IS_GEN(dev_priv, 2) || crtc_state->active_planes)
 		intel_set_cpu_fifo_underrun_reporting(dev_priv, crtc->pipe, true);
 
 	if (crtc_state->has_pch_encoder) {