drm/vgem: Remove unecessary dma_fence_ops

drm/vgem: Remove unecessary dma_fence_ops

dma_fence_default_wait is the default now, same for the trivial
enable_signaling implementation.

Also remove the ->signaled callback, vgem can't peek ahead with a
fastpath, returning false is the default implementation.

v2: Protect the meaningful space! (Chris)

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Kees Cook <keescook@chromium.org>
Cc: Cihangir Akturk <cakturk@gmail.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180809124544.9250-1-daniel.vetter@ffwll.ch