drm/xe: flush engine buffers before signalling user fence on all engines

Tests show that user fence signalling requires kind of write barrier,
otherwise not all writes performed by the workload will be available
to userspace. It is already done for render and compute, we need it
also for the rest: video, gsc, copy.

Fixes: dd08ebf6 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240605-fix_user_fence_posted-v3-2-06e7932f784a@intel.com