configure.ac: list all the dri-drivers in the help string

It's unlikely that any of the additions come as a suprise to anyone
i915, nouveau, radeon, r200. Regardless, state clearly what's
available.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Chad Versace <chadversary@chromium.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
diff --git a/configure.ac b/configure.ac
index 7cb2c40..0addfd5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1621,7 +1621,7 @@
 AC_ARG_WITH([dri-drivers],
     [AS_HELP_STRING([--with-dri-drivers@<:@=DIRS...@:>@],
         [comma delimited classic DRI drivers list, e.g.
-        "swrast,i965,radeon" @<:@default=auto@:>@])],
+        "i915,i965,nouveau,radeon,r200,swrast" @<:@default=auto@:>@])],
     [with_dri_drivers="$withval"],
     [with_dri_drivers=auto])