freedreno: implement fd_screen_destroy()
freedreno: implement fd_screen_destroy()

Opps, didn't notice that I had left it stubbed out.

Also, make things fail a bit more gracefully when things go wrong.

Signed-off-by: Rob Clark <6f78cdb10adbe0a7fbbf4098125a51f401c77a5f@freedesktop.org>

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