Remove a few GrSurfaceProxy::config() usages

Change-Id: I9da9e1dadea71701c97994f8df1e7b5912989672
Bug: skia:6718
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/243159
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/src/shaders/SkImageShader.cpp b/src/shaders/SkImageShader.cpp
index ef4e163..f263a2d 100644
--- a/src/shaders/SkImageShader.cpp
+++ b/src/shaders/SkImageShader.cpp
@@ -228,8 +228,7 @@
         return nullptr;
     }
 
-    GrPixelConfig config = proxy->config();
-    bool isAlphaOnly = GrPixelConfigIsAlphaOnly(config);
+    bool isAlphaOnly = SkColorTypeIsAlphaOnly(fImage->colorType());
 
     lmInverse.postScale(scaleAdjust[0], scaleAdjust[1]);