lib: add igt_restore_vt_mode

Add a function to restore the previous VT mode after
igt_set_vt_graphics_mode is called.

Signed-off-by: Thomas Wood <thomas.wood@intel.com>

lib/igt_kms.h

@@ -102,6 +102,7 @@ void kmstest_dump_mode(drmModeModeInfo *mode);
 
 int kmstest_get_pipe_from_crtc_id(int fd, int crtc_id);
 void kmstest_set_vt_graphics_mode(void);
+void kmstest_restore_vt_mode(void);
 
 struct kmstest_connector_config {
 	drmModeCrtc *crtc;