configure.ac: fix up the help text for --enable-gallium-r300

configure.ac

@@ -1775,7 +1775,7 @@ dnl Gallium Radeon r300g configuration
 dnl
 AC_ARG_ENABLE([gallium-r300],
     [AS_HELP_STRING([--enable-gallium-r300],
-        [build gallium r300 @<:@default=disabled@:>@])],
+        [build gallium r300 @<:@default=DRI-only@:>@])],
     [enable_gallium_r300="$enableval"],
     [enable_gallium_r300=auto])
 if test "x$enable_gallium_r300" = xauto; then