drm/qxl: use DEFINE_DRM_GEM_FOPS()

We have no qxl-specific fops any more.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20191017132638.9693-5-kraxel@redhat.com