rtsp-media: Force seek when flush flag is set

The commit "rtsp-client: define all seek accuracy flags from
setup_play_mode" changed the behaviour of when doing a seek.

Before that commit, having the flush flag set would result in a seek
(forced seek).
Even if no seek was needed. One reason to force seek is to flush old buffers
created in Describe requests.

Thus adding force seek also for flush flag will result in play request
with fresh buffers.
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