drm/i915: Register shadow VGA even when it produces spurious detection results

Having a shadow VGA connector is useful for testing purposes. We
currently skip registering the connector on machines where the
CRT detect falsely reports it as connected. Let's instead move the
the blacklist check to the detect callback (and hpd setup) and
if we get a match we always report the connector as disconnected.
This way we get a shadow VGA connector to help with testing, while
we still avoid the user facing problems from the incorrect
detection results.

commit 8ca4013d

 ("CHROMIUM: i915: Add DMI override to skip CRT
initialization on ZGB") doesn't provide much in the way of details
as to why 'ACER ZGB' was added to the blacklist. Trying to trace it
further leads me to a chromeos bugreport I can't access. So based on
the fact that the commit added the
"/* Skip machines without VGA that falsely report hotplug events */"
comment, I'm going to assume that it was just spurious CRT detection.
So it should be safe to move the blacklist to just block the detection
and hpd without causing a regression on said machine.

In fact Stéphane confirmed on irc that the problem was indeed just
crappy hotplug detect:
"22:29 < marcheu> vsyrjala: the port isn't there, but the load detect is
 improperly stubbed in hw
 22:29 < marcheu> vsyrjala: so it floats"
so this change should be perfectly fine.

v2: Add irc quote from Stéphane

Cc: Duncan Laurie <dlaurie@chromium.org>
Cc: Olof Johansson <olofj@chromium.org>
Cc: Stéphane Marchesin <marcheu@chromium.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474881646-1326-2-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>