drm/nouveau/kms/gt215-: fix race with audio driver runpm

The audio driver can call into nouveau right while we're in the middle
of re-fetching the EDID, and decide it no longer needs to be awake.

Stop depending on EDID in the audio component get_eld() callback, and
instead cache whether audio support is present from the prior modeset.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>