configure.ac: default to disabled dri3 when --disable-dri is set

Not too long ago, the dri3 code was living in src/glx, which in itself
was guarded by HAVE_DRI_GLX. As the name suggests we didn't dive into
the folder when dri was disabled, thus we missed that dri3 does not
consider/honour --enable-dri.

Cc: mesa-stable@lists.freedesktop.org
Fixes: 6bd9ba7d07 "loader: Add dri3 helper"
Cc: Pali Rohár <pali.rohar@gmail.com>
Reported-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

configure.ac

@@ -767,6 +767,11 @@ linux*)
     dri3_default=no
     ;;
 esac
+
+if test "x$enable_dri" = xno; then
+    dri3_default=no
+fi
+
 AC_ARG_ENABLE([dri3],
     [AS_HELP_STRING([--enable-dri3],
         [enable DRI3 @<:@default=auto@:>@])],