drm-smoke-test: Explicitly set-up the output name DRM test

In case we have multiple outputs let's us choose the first output,
rather use the last one, which would happen when multiple are present in
the system.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

tests/drm-smoke-test.c

@@ -90,7 +90,7 @@ TEST(drm_screenshot_no_damage) {
 	 */
 	for (i = 0; i < 5; i++) {
 		ret = verify_screen_content(client, "drm_screenshot_no_damage",
-					    0, NULL, i, NULL);
+					    0, NULL, i, "Virtual-1");
 		assert(ret);
 	}