ui/gtk: fix passing y0_top parameter to scanout

ui/gtk: fix passing y0_top parameter to scanout

The dmabuf->y0_top flag is passed to .dpy_gl_scanout_dmabuf(), however
in the gtk ui both implementations dropped it when doing the next
scanout_texture call.

Fixes flipped linux console using vhost-user-gpu with the gtk ui
display.

Signed-off-by: Erico Nunes <ernunes@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230220175605.43759-1-ernunes@redhat.com>
(cherry picked from commit 94400fa53f81c9f58ad88cf3f3e7ea89ec423d39)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>