drm/fbdev: rework output polling to be back in the core. (v4)

After thinking it over a lot it made more sense for the core to deal with
the output polling especially so it can notify X.

v2: drop plans for fake connector - per Michel's comments - fix X patch sent to xorg-devel, add intel polled/hpd setting, add initial nouveau polled/hpd settings.

v3: add config lock take inside polling, add intel/nouveau poll init/fini calls

v4: config lock was a bit agressive, only needed around connector list reading.
otherwise it could re-enter.

glisse: discard drm_helper_hpd_irq_event

v3: Reviewed-by: Michel Dänzer <michel@daenzer.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>