zink: do not unmap dt-buffers twice

Seems I missed that we already did an unconditional unmap here, and
forgot to remove it. Whoops.

Fixes: 5159f406d ("zink: use gallium api to copy to display-target")
Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11414>

src/gallium/drivers/zink/zink_screen.c

@@ -1132,8 +1132,6 @@ zink_flush_frontbuffer(struct pipe_screen *pscreen,
       winsys->displaytarget_unmap(winsys, res->dt);
    }
 
-   winsys->displaytarget_unmap(winsys, res->dt);
-
    assert(res->dt);
    if (res->dt)
       winsys->displaytarget_display(winsys, res->dt, winsys_drawable_handle, sub_box);