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/1467297225-21379-28-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_dp_mst.c

@@ -213,7 +213,10 @@ static void intel_mst_enable_dp(struct intel_encoder *encoder)
 
 	DRM_DEBUG_KMS("%d\n", intel_dp->active_mst_links);
 
-	if (wait_for((I915_READ(DP_TP_STATUS(port)) & DP_TP_STATUS_ACT_SENT),
+	if (intel_wait_for_register(dev_priv,
+				    DP_TP_STATUS(port),
+				    DP_TP_STATUS_ACT_SENT,
+				    DP_TP_STATUS_ACT_SENT,
 				    1))
 		DRM_ERROR("Timed out waiting for ACT sent\n");