etnaviv: Unmap gems on gem_close

This fixes the unplug/replug corruptions (phoc#164 (closed)) while not breaking any of the games i tested. These usually manifest as a borken internal display and

[E] submit failed: -14 (No space left on device) (etna_cmd_stream_flush:244)

If that's not in the log it's another problem

I did most of the tests with mesa muster but also with our current version in amber-phone.

(i'll add a link to the upstream submission once mailman catched up)

Closes: phoc#164 (closed)