drm/i915: Revert extra intel_wait_for_vblank to prevent stalls.

With the extra intel_wait_for_vblank added in commit
9d0498a2 periodic stalls were being
triggered (which were detected by i915_hangcheck_elapsed). Partially
revert this change for now.
Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_display.c

@@ -2328,8 +2328,10 @@ static void i9xx_crtc_dpms(struct drm_crtc *crtc, int mode)
 			I915_READ(dspbase_reg);
 		}
 
+		if (!IS_I9XX(dev)) {
 			/* Wait for vblank for the disable to take effect */
 			intel_wait_for_vblank_off(dev, pipe);
+		}
 
 		/* Don't disable pipe A or pipe A PLLs if needed */
 		if (pipeconf_reg == PIPEACONF &&