drm/qxl: Remove unecessary dma_fence_ops

The trivial enable_signaling implementation matches the default code.

v2: Fix up commit message to match patch better (Eric).

Cc: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Link: https://patchwork.freedesktop.org/patch/msgid/20180502082325.30264-1-daniel.vetter@ffwll.ch