use colortype instead of config

clone of https://codereview.chromium.org/305133006/

TBR=

BUG=skia:

Author: reed@google.com

Review URL: https://codereview.chromium.org/301233011
diff --git a/src/effects/SkDisplacementMapEffect.cpp b/src/effects/SkDisplacementMapEffect.cpp
index d0f134b..a5519d1 100644
--- a/src/effects/SkDisplacementMapEffect.cpp
+++ b/src/effects/SkDisplacementMapEffect.cpp
@@ -227,8 +227,7 @@
         return false;
     }
 
-    dst->setConfig(color.config(), bounds.width(), bounds.height());
-    if (!dst->allocPixels()) {
+    if (!dst->allocPixels(color.info().makeWH(bounds.width(), bounds.height()))) {
         return false;
     }