intel: Fix up math errors when allocating very large BOs.

The logbase2 would overflow and wrap the size around to 0, making the code
allocate a 4kb object instead.  By simplifying the code to just walk the
14-entry bucket array comparing sizes instead of indexing on
ffs(1 << logbase2(size)), we avoid silly math errors and have code of
approximately the same speed.

Many thanks to Simon Farnsworth for debugging and providing a working patch.
Bug #27365.