configure.ac: dump dri2proto requirement to 2.8

dri2proto 2.8 was released 4+ years ago, so it must be of no surprise
for anyone building mesa from git.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
diff --git a/configure.ac b/configure.ac
index 39da5b4..d0c3b55 100644
--- a/configure.ac
+++ b/configure.ac
@@ -83,7 +83,7 @@
 LIBDRM_ETNAVIV_REQUIRED=2.4.74
 
 dnl Versions for external dependencies
-DRI2PROTO_REQUIRED=2.6
+DRI2PROTO_REQUIRED=2.8
 DRI3PROTO_REQUIRED=1.0
 PRESENTPROTO_REQUIRED=1.0
 GLPROTO_REQUIRED=1.4.14