mesa: Add functions for doing unlocked hash table lookups

mesa: Add functions for doing unlocked hash table lookups

This patch adds functions for locking/unlocking the mutex, along with
_mesa_HashLookupLocked() and _mesa_HashInsertLocked()
that do lookups and insertions without locking the mutex.

These functions will be used by the ARB_multi_bind entry points to
avoid locking/unlocking the mutex for each binding point.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>