drm/i915: Assume non-DP++ port if dvo_port is HDMI and there's no AUX ch specified...

drm/i915: Assume non-DP++ port if dvo_port is HDMI and there's no AUX ch specified in the VBT

My heuristic for detecting type 1 DVI DP++ adaptors based on the VBT
port information apparently didn't survive the reality of buggy VBTs.
In this particular case we have a machine with a natice HDMI port, but
the VBT tells us it's a DP++ port based on its capabilities.

The dvo_port information in VBT does claim that we're dealing with a
HDMI port though, but we have other machines which do the same even
when they actually have DP++ ports. So that piece of information alone
isn't sufficient to tell the two apart.

After staring at a bunch of VBTs from various machines, I have to
conclude that the only other semi-reliable clue we can use is the
presence of the AUX channel in the VBT. On this particular machine
AUX channel is specified as zero, whereas on some of the other machines
which listed the DP++ port as HDMI have a non-zero AUX channel.

I've also seen VBTs which have dvo_port a DP but have a zero AUX
channel. I believe those we need to treat as DP ports, so we'll limit
the AUX channel check to just the cases where dvo_port is HDMI.

If we encounter any more serious failures with this heuristic I think
we'll have to have to throw it out entirely. But that could mean that
there is a risk of type 1 DVI dongle users getting greeted by a
black screen, so I'd rather not go there unless absolutely necessary.

v2: Remove the duplicate PORT_A check (Daniel)
    Fix some typos in the commit message

Cc: Daniel Otero <daniel.otero@outlook.com>
Cc: stable@vger.kernel.org
Tested-by: Daniel Otero <daniel.otero@outlook.com>
Fixes: d61992565bd3 ("drm/i915: Determine DP++ type 1 DVI adaptor presence based on VBT")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97994
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1478884464-14251-1-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>