mesa: re-enable _mesa_source_buffer_exists() call

Somehow this code wound up inside a comment a while back.

src/mesa/main/teximage.c

@@ -1869,14 +1869,14 @@ copytexture_error_check( GLcontext *ctx, GLuint dimensions,
       return GL_TRUE;
    }
 
-   /* NOTE: the format and type aren't really significant for
-    * TestProxyTexImage().  Only the internalformat really matters.
    if (!_mesa_source_buffer_exists(ctx, format)) {
       _mesa_error(ctx, GL_INVALID_OPERATION,
                   "glCopyTexImage%dD(missing readbuffer)", dimensions);
       return GL_TRUE;
    }
 
+   /* NOTE: the format and type aren't really significant for
+    * TestProxyTexImage().  Only the internalformat really matters.
     */
    type = GL_FLOAT;