backend-drm: skip EDID parsing if no change

Performance-wise this is moot, but since we are detecting if the raw
EDID data changed, might as well use it.

Now we have a path where the head's device_changed is almost guaranteed
since EDID changed, and that's useful for the next patch.

We can only do this, because the core initializes a head with values
that we would be setting anyway when EDID is missing, e.g. disconnected
head on compositor start-up.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>