Revert "Disable HAVE_DRM_ATOMIC."

This reverts commit 87bee72d15955a8acee5450e08d9e08b7ec2423e.

Change-Id: Ied7b22c0dd9494f79570028c5824a0d48e781b06
diff --git a/configure.ac b/configure.ac
index b1162a1..9d98e84 100644
--- a/configure.ac
+++ b/configure.ac
@@ -233,9 +233,9 @@
   PKG_CHECK_MODULES(DRM_COMPOSITOR_MODIFIERS, [libdrm >= 2.4.71],
 		    [AC_DEFINE([HAVE_DRM_ADDFB2_MODIFIERS], 1, [libdrm supports modifiers])],
 		    [AC_MSG_WARN([libdrm does not support AddFB2 with modifiers])])
-  dnl PKG_CHECK_MODULES(DRM_COMPOSITOR_ATOMIC, [libdrm >= 2.4.78],
-	dnl 	    [AC_DEFINE([HAVE_DRM_ATOMIC], 1, [libdrm supports atomic API])],
-	dnl 	    [AC_MSG_WARN([libdrm does not support atomic modesetting, will omit that capability])])
+  PKG_CHECK_MODULES(DRM_COMPOSITOR_ATOMIC, [libdrm >= 2.4.78],
+		    [AC_DEFINE([HAVE_DRM_ATOMIC], 1, [libdrm supports atomic API])],
+		    [AC_MSG_WARN([libdrm does not support atomic modesetting, will omit that capability])])
   PKG_CHECK_MODULES(DRM_COMPOSITOR_FORMATS_BLOB, [libdrm >= 2.4.83],
 		    [AC_DEFINE([HAVE_DRM_FORMATS_BLOB], 1, [libdrm supports modifier advertisement])],
 		    [AC_MSG_WARN([libdrm does not support modifier advertisement])])