dix: cache ResourceClientBits() value
The LimitClient is set once and for all at startup, whereas the
function ResourceClientBits() which returns the client field offset
within the XID based on the value of LimitClient can be called
repeatedly.
Small optimization, cache the result of ilog2(), that saves running
the same loop over and over each time ResourceClientBits() is called.
Signed-off-by: Olivier Fourdan ofourdan@redhat.com