Asahi DCP: HDMI audio dies after monitor reconnect

Update (Sep 24): this fix is real and still correct for what it addresses, but it turned out not to be the whole story — after ~16 reconnect cycles I hit a second, different bug in the same code path.

AI used to find and fix this bug.

Every time I turned my monitor off and back on, HDMI audio died and never came back until I rebooted. Turns out a flag never got reset after the first disconnect. Two-line fix in drivers/gpu/drm/apple/av.c.

Tested on Mac mini M2, Debian (Bananas), kernel 7.1.13 — 3 TV power-cycles, audio survived all 3 (previously died after the first).