kmssink: Add support for the Allwinner DRM driver (sun4i-drm)

This adds the sun4i DRM driver to the list of DRM drivers in kmssink.
The driver allows displaying video in either the main plane or an
overlay plane.

https://bugzilla.gnome.org/attachment.cgi?bugid=794839
diff --git a/sys/kms/gstkmssink.c b/sys/kms/gstkmssink.c
index 1b5f18d..4991ee3 100644
--- a/sys/kms/gstkmssink.c
+++ b/sys/kms/gstkmssink.c
@@ -168,7 +168,7 @@
 {
   static const char *drivers[] = { "i915", "radeon", "nouveau", "vmwgfx",
     "exynos", "amdgpu", "imx-drm", "rockchip", "atmel-hlcdc", "msm",
-    "xilinx_drm", "vc4", "meson",
+    "xilinx_drm", "vc4", "meson", "sun4i-drm",
   };
   int i, fd = -1;