Remove GrIsPow2 in favor of SkIsPow2.

Looks like there is no good reason to have two copies of this function
doing the same thing with different name.

BUG=None
TEST=make tests
R=bsalomon@google.com

Author: tfarina@chromium.org

Review URL: https://codereview.chromium.org/318873002
diff --git a/src/gpu/GrContext.cpp b/src/gpu/GrContext.cpp
index 66588c4..e8ae18a 100644
--- a/src/gpu/GrContext.cpp
+++ b/src/gpu/GrContext.cpp
@@ -618,7 +618,7 @@
         return false;
     }
 
-    bool isPow2 = GrIsPow2(width) && GrIsPow2(height);
+    bool isPow2 = SkIsPow2(width) && SkIsPow2(height);
 
     if (!isPow2) {
         bool tiled = NULL != params && params->isTiled();