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

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 0bf683d..2a32a7b 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -1665,6 +1665,7 @@
 	dpa_ctl = I915_READ(DP_A);
 	dpa_ctl |= DP_PLL_ENABLE;
 	I915_WRITE(DP_A, dpa_ctl);
+	POSTING_READ(DP_A);
 	udelay(200);
 }