drm/i915/crt: Flush register prior to waiting for vblank.

If we don't flush the write then we can not be sure that the border
colour will have taken effect by the time we try to read it back.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_crt.c

@@ -327,6 +327,7 @@ intel_crt_load_detect(struct drm_crtc *crtc, struct intel_encoder *intel_encoder
 	if (IS_I9XX(dev)) {
 		uint32_t pipeconf = I915_READ(pipeconf_reg);
 		I915_WRITE(pipeconf_reg, pipeconf | PIPECONF_FORCE_BORDER);
+		POSTING_READ(pipeconf_reg);
 		/* Wait for next Vblank to substitue
 		 * border color for Color info */
 		intel_wait_for_vblank(dev, pipe);