drm/i915/edp: Flush the write before waiting for PLLs

commit 5ddb954b upstream.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/gpu/drm/i915/intel_display.c

@@ -1402,6 +1402,7 @@ static void igdng_enable_pll_edp (struct drm_crtc *crtc)
 	dpa_ctl = I915_READ(DP_A);
 	dpa_ctl |= DP_PLL_ENABLE;
 	I915_WRITE(DP_A, dpa_ctl);
+	POSTING_READ(DP_A);
 	udelay(200);
 }