glx: Add an optional function call for getting the DRI driver interface.

glx: Add an optional function call for getting the DRI driver interface.

The previous interface relied on a static struct, which meant that the
driver didn't get a chance to edit the struct before the struct got used.
For megadrivers, I want struct specific to the driver being loaded.

v2: Fix the prototype in the docs (caught by Marek).  Since the driver
    name was in the function, we didn't need to also pass it in.
v3: Fix asprintf error checking (caught by Matt's gcc).

Reviewed-by: Matt Turner <mattst88@gmail.com> (v1)
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>