drm/i915: Check VBT for port presence in addition to the strap on VLV/CHV

Apparently some CHV boards failed to hook up the port presence straps
for HDMI ports as well (earlier we assumed this problem only affected
eDP ports). So let's check the VBT in addition to the strap, and if
either one claims that the port is present go ahead and register the
relevant connector.

While at it, change port D to register DP before HDMI as we do for ports
B and C since
commit 457c52d87e5d ("drm/i915: Only ignore eDP ports that are connected")

Also print a debug message when we register a HDMI connector to aid
in diagnosing missing/incorrect ports. We already had such a print for
DP/eDP.

v2: Improve the comment in the code a bit, note the port D change in
    the commit message

Cc: Radoslav Duda <radosd@radosd.com>
Tested-by: Radoslav Duda <radosd@radosd.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96321
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464945463-14364-1-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>