drm/i915/dp: Flush the PLL register write before sleeping

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index 27805a9..c7aa29b 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -883,6 +883,7 @@
 	dpa_ctl = I915_READ(DP_A);
 	dpa_ctl |= DP_PLL_ENABLE;
 	I915_WRITE(DP_A, dpa_ctl);
+	POSTING_READ(DP_A);
 	udelay(200);
 }