mesa: do not use _glapi_new_nop_table() for DRI builds

mesa: do not use _glapi_new_nop_table() for DRI builds

Commit 4bdbb588a9d38 introduced new _glapi_new_nop_table() and
_glapi_set_nop_handler() functions in the glapi dispatcher (which
live in libGL.so).  The calls to those functions from context.c
would be undefined (i.e. an ABI break) if the libGL used at runtime
was older.

For the time being, use the old single generic_nop() function for
non-Windows builds to avoid this problem.  At some point in the future
it should be safe to remove this work-around.  See comments for more
details.

v2: Incorporate feedback from Emil.  Use _WIN32 instead of
GLX_DIRECT_RENDERING to control behavior, move comments.

Cc: 10.6 <mesa-stable@lists.freedesktop.org>
Reviewed-and-tested-by: Ian Romanick <ian.d.romanick@intel.com>