util: Add util_unsigned_logbase2.

Cherry picked from Keith's commit f911c3b989.

src/gallium/auxiliary/util/u_math.h

@@ -375,6 +375,18 @@ unsigned ffs( unsigned u )
 #endif
 
 
+/* Could also binary search for the highest bit.
+ */
+static INLINE unsigned
+util_unsigned_logbase2(unsigned n)
+{
+   unsigned log2 = 0;
+   while (n >>= 1)
+      ++log2;
+   return log2;
+}
+
+
 /**
  * Return float bits.
  */