build, compositor-drm: fix output name constants another way

Bumping libdrm requirement by 3 years just for output connector name
constants was a bit much. Fix the problem introduced in
89c49b3060a115e846ba1e7fbef94d14894244f2 by conditionally using the new
additions.

Both VIRTUAL and DSI came in the same libdrm commit
566c3ce877a4be72697e15cdfc421ce965f7c37d, so we check only for DSI.

This patch also reverts faee330c5e027011941d50533156683be3a67ca3.

Reported-by: Eugen Friedrich <friedrix@gmail.com>
Cc: Derek Foreman <derekf@osg.samsung.com>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
Acked-by: Bryce Harrington <bryce@osg.samsung.com>
diff --git a/src/compositor-drm.c b/src/compositor-drm.c
index c85a462..f5f125a 100644
--- a/src/compositor-drm.c
+++ b/src/compositor-drm.c
@@ -1760,8 +1760,10 @@
 	[DRM_MODE_CONNECTOR_HDMIB]       = "HDMI-B",
 	[DRM_MODE_CONNECTOR_TV]          = "TV",
 	[DRM_MODE_CONNECTOR_eDP]         = "eDP",
+#ifdef DRM_MODE_CONNECTOR_DSI
 	[DRM_MODE_CONNECTOR_VIRTUAL]     = "Virtual",
 	[DRM_MODE_CONNECTOR_DSI]         = "DSI",
+#endif
 };
 
 static char *