configure: update help strings

Mention that freedreno and intel are both auto-detected (depending on
the presence of atomics), plus host_cpu "defined".

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
diff --git a/configure.ac b/configure.ac
index 6300083..437f4da 100644
--- a/configure.ac
+++ b/configure.ac
@@ -60,7 +60,7 @@
 
 AC_ARG_ENABLE(libkms,
 	      AS_HELP_STRING([--disable-libkms],
-	      [Disable KMS mm abstraction library (default: auto)]),
+	      [Disable KMS mm abstraction library (default: auto, enabled on x86]),
 	      [LIBKMS=$enableval], [LIBKMS=auto])
 
 AC_ARG_ENABLE(intel,
@@ -95,7 +95,7 @@
 
 AC_ARG_ENABLE(freedreno,
 	      AS_HELP_STRING([--disable-freedreno],
-	      [Enable support for freedreno's KMS API (default: enabled on arm)]),
+	      [Enable support for freedreno's KMS API (default: auto, enabled on arm)]),
 	      [FREEDRENO=$enableval], [FREEDRENO=auto])
 
 AC_ARG_ENABLE(freedreno-kgsl,