Fix compiler warnings generated by older toolchains/compiler

This fixes warnings like ``may be used uninitialized''

libweston/compositor-drm.c: In function 'drm_device_is_kms':
libweston/compositor-drm.c:6374:12: warning: 'id' may be used uninitialized in this function [-Wmaybe-uninitialized]
  b->drm.id = id;

Signed-off-by: Marius Vlad <marius.vlad0@gmail.com>

libweston/compositor-drm.c

@@ -6680,7 +6680,7 @@ drm_device_is_kms(struct drm_backend *b, struct udev_device *device)
 	const char *filename = udev_device_get_devnode(device);
 	const char *sysnum = udev_device_get_sysnum(device);
 	drmModeRes *res;
-	int id, fd;
+	int id = -1, fd;
 
 	if (!filename)
 		return false;