drm/vmwgfx: Fix a sparse warning in kernel docs

Fixes a warning about extra docs about a function argument that has been
removed a while back:
drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c:3888: warning: Excess function
parameter 'sync_file' description in 'vmw_execbuf_copy_fence_user'

Fixes: a0f90c88 ("drm/vmwgfx: Fix stale file descriptors on failed usercopy")
Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Reviewed-by: Maaz Mombasawala <mombasawalam@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221022040236.616490-18-zack@kde.org