tests: add virtio_gpu to the driver list

modetest was failing to work with driver because it wasn't in the
module list.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
diff --git a/tests/util/kms.c b/tests/util/kms.c
index ce8aaab..650b23b 100644
--- a/tests/util/kms.c
+++ b/tests/util/kms.c
@@ -141,6 +141,7 @@
 	"atmel-hlcdc",
 	"fsl-dcu-drm",
 	"vc4",
+	"virtio_gpu",
 };
 
 int util_open(const char *device, const char *module)