glx: load dri driver with RTLD_LOCAL so dlclose never fails to unload

glx: load dri driver with RTLD_LOCAL so dlclose never fails to unload

There is no reason anymore to load with RTLD_GLOBAL and for some driver
this even result in dlclose failing to unload leading to catastrophic
failure with swrast fallback.

Cc: "10.1 10.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Jérôme Glisse <jglisse@redhat.com>