lib: make igt_set_vt_graphics_mode never fail

All tests agree that this is a letal failure, so no point
to pass it back to callers.

Also add some igt_require calls for the drm_fd where I've
noticed that it's missing.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

tests/testdisplay.c

@@ -801,7 +801,7 @@ int main(int argc, char **argv)
 		goto out_close;
 	}
 
-	do_or_die(igt_set_vt_graphics_mode());
+	igt_set_vt_graphics_mode();
 
 	mainloop = g_main_loop_new(NULL, FALSE);
 	if (!mainloop) {