drm/i915: Add missing NULL check before calling initial_watermarks

Not all platforms set this callback, so NULL check it before calling it.

v2:
- Call intel_update_watermarks() on HSW+ where the callback is not set.
  (Matt)

CC: Matt Roper <matthew.d.roper@intel.com>
Fixes: commit ed4a6a7c ("drm/i915: Add two-stage ILK-style watermark programming (v11)")
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456776633-3401-1-git-send-email-imre.deak@intel.comReviewed-by: Matt Roper <matthew.d.roper@intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -4957,6 +4957,7 @@ static void ironlake_crtc_enable(struct drm_crtc *crtc)
 	 */
 	intel_crtc_load_lut(crtc);
 
+	if (dev_priv->display.initial_watermarks != NULL)
 		dev_priv->display.initial_watermarks(intel_crtc->config);
 	intel_enable_pipe(intel_crtc);
 
@@ -5056,7 +5057,10 @@ static void haswell_crtc_enable(struct drm_crtc *crtc)
 	if (!intel_crtc->config->has_dsi_encoder)
 		intel_ddi_enable_transcoder_func(crtc);
 
+	if (dev_priv->display.initial_watermarks != NULL)
 		dev_priv->display.initial_watermarks(pipe_config);
+	else
+		intel_update_watermarks(crtc);
 	intel_enable_pipe(intel_crtc);
 
 	if (intel_crtc->config->has_pch_encoder)