mesa: Don't pass paramter to __builtin_clz which would hve undefined result.

mesa: Don't pass paramter to __builtin_clz which would hve undefined result.

__builtin_clz with parameter 0 has undefined value. When
using -O3 optimizing this would result to too large next
power of two value.

Fix is to check if passed value is 1 and modify formula
for that case.