drm/nouveau/disp: add conn method to query HPD pin status

drm/nouveau/disp: add conn method to query HPD pin status

And use it to bail early in DP detection and avoid futile AUX transactions.

This could be used on other connector types too in theory, but it's not
something we've ever done before and I'd rather not risk breaking working
systems without looking into it more closely.

It's safe for DP though.  We already do this by checking an AUX register
that contains HPD status and aborting the transaction.  However, this is
much deeper in the stack - after taking various mutexes, poking HW for no
good reason, and making a mess in debug logs.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>