drm/msm/atomic: fix issue with gnome-shell wayland

The gnome-shell wayland compositor triggers a setcrtc with an fb that is
still being rendered, triggering the call to _wait_fence_interruptable().
But a NULL timeout means "don't wait, return -EBUSY if not ready", which
in turn causes the setcrtc to fail.

Signed-off-by: Rob Clark <robdclark@gmail.com>