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>