drm/i915: Convert wait_for(I915_READ(reg)) to intel_wait_for_register()

By using the out-of-line intel_wait_for_register() not only do we can
efficiency from using the hybrid wait_for() contained within, but we
avoid code bloat from the numerous inlined loops, in total (all patches):

   text    data     bss     dec     hex filename
1078551    4557     416 1083524  108884 drivers/gpu/drm/i915/i915.ko
1070775    4557     416 1075748  106a24 drivers/gpu/drm/i915/i915.ko

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467297225-21379-47-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -529,7 +529,11 @@ static bool stop_ring(struct intel_engine_cs *engine)
 
 	if (!IS_GEN2(dev_priv)) {
 		I915_WRITE_MODE(engine, _MASKED_BIT_ENABLE(STOP_RING));
-		if (wait_for((I915_READ_MODE(engine) & MODE_IDLE) != 0, 1000)) {
+		if (intel_wait_for_register(dev_priv,
+					    RING_MI_MODE(engine->mmio_base),
+					    MODE_IDLE,
+					    MODE_IDLE,
+					    1000)) {
 			DRM_ERROR("%s : timed out trying to stop ring\n",
 				  engine->name);
 			/* Sometimes we observe that the idle flag is not