configure.ac: Lower require dri2proto version to 2.1

Additions after 2.1 are implemented in a conditional way and will be
supported if available.

configure.ac

@@ -20,7 +20,7 @@ AC_CANONICAL_HOST
 dnl Versions for external dependencies
 LIBDRM_REQUIRED=2.4.15
 LIBDRM_RADEON_REQUIRED=2.4.17
-DRI2PROTO_REQUIRED=2.2
+DRI2PROTO_REQUIRED=2.1
 GLPROTO_REQUIRED=1.4.11
 
 dnl Check for progs