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>

lib/drmtest.h

@@ -394,7 +394,7 @@ void igt_enable_exit_handler(void);
 void igt_disable_exit_handler(void);
 
 /* set vt into graphics mode, required to prevent fbcon from interfering */
-int igt_set_vt_graphics_mode(void);
+void igt_set_vt_graphics_mode(void);
 
 /* prefault disabling, needs the corresponding debugfs interface */
 void igt_disable_prefault(void);