driDestroyDisplay: Remove Drivers list entry when dlclosing its handle.

driDestroyDisplay: Remove Drivers list entry when dlclosing its handle.

This fixes a regression from commit f81b1dbe374fe446f6ef676e70a72952ffb47d4e:
Since then, driDestroyDisplay gets called from __glXFreeDisplayPrivate. It
dlcloses the handles associated with the display but fails to remove their
references from the Drivers list, so subsequent calls to OpenDriver return a
stale handle and an invalid createNewScreenFunc pointer. The attempt to call
the latter results in a segfault when running amoeba, e.g.