drm/i915: Suppress spurious underruns on gen2

Often we seem to detect an underrun right after modeset on gen2.
It seems to be a spurious detection (potentially the pipe is still
in a wonky state when we enable the planes). An extra vblank wait
seems to cure it.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200429101034.8208-13-ville.syrjala@linux.intel.comReviewed-by: José Roberto de Souza <jose.souza@intel.com>

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

@@ -7526,6 +7526,10 @@ static void i9xx_crtc_enable(struct intel_atomic_state *state,
 	intel_crtc_vblank_on(new_crtc_state);
 
 	intel_encoders_enable(state, crtc);
+
+	/* prevents spurious underruns */
+	if (IS_GEN(dev_priv, 2))
+		intel_wait_for_vblank(dev_priv, pipe);
 }
 
 static void i9xx_pfit_disable(const struct intel_crtc_state *old_crtc_state)