nouveau: Fix crash when destination or source screen fences are null
What does this MR do and why?
nouveau: Fix crash when destination or source screen fences are null
Fixes: dEQP-EGL.functional.sharing.gles2.multithread.random_egl_sync.*,
one of them, its quite finiky, one may say random
Signed-off-by: Yusuf Khan <yusisamerican@gmail.com>