Try and find the active connector.

listconnectors.c

/* gcc -o listconnectors listconnectors.c $(pkg-config --cflags --libs glib-2.0 libdrm) */

#include <glib.h>
#include <fcntl.h>
#include <stdlib.h>
#include <xf86drm.h>
#include <xf86drmMode.h>

int
main (int argc, char **argv)
{
  int i;
  int fd;
  drmModeRes *resources = NULL;
  drmModeConnector *connector = NULL;

  fd = open ("/dev/dri/card0", O_RDWR);
  if (fd < 0)
    {
      g_warning ("Unable to open DRI device");
      exit (1);
    }

  resources = drmModeGetResources (fd);

  g_print ("There are %d connectors in total\n", resources->count_connectors);

  /* Find the first active connector to display on. */
  for (i = 0; i < resources->count_connectors; i++)
    {
      connector = drmModeGetConnector (fd, resources->connectors[i]);
      if (connector == NULL)
        continue;

      if (connector->connection == DRM_MODE_CONNECTED &&
          connector->count_modes > 0)
        break;

      drmModeFreeConnector(connector);
    }

  if (i == resources->count_connectors)
    {
      g_warning ("Could not find an active connector");
      exit (1);
    }

  g_print ("The active connector is %d, of type %d\n",
           connector->connector_id, connector->connector_type);
  return 0;
}