I found out that elsewhere in the drm code (e.g. in drm_atomic_helper.c) expects drm_vblank_get() to fail as part of normal operation. So this patch doesn't seem appropriate anymore and it seems more appropriate to hunt down all the unchecked calls for drm_vblank_get
and fix them instead. I don't know how to best make sure this doesn't reoccur in the future though.