DP dual mode type 1 DVI adaptors aren't required to implement any
registers, so it's a bit hard to detect them. The best way would
be to check the state of the CONFIG1 pin, but we have no way to
do that. So as a last resort, check the VBT to see if the HDMI
port is in fact a dual mode capable DP port.

v2: Deal with VBT code reorganization
    Deal with DRM_DP_DUAL_MODE_UNKNOWN
    Reduce DEVICE_TYPE_DP_DUAL_MODE_BITS a bit
    Accept both DP and HDMI dvo_port in VBT as my BSW
    at least declare its DP port as HDMI :(
v3: Ignore DEVICE_TYPE_NOT_HDMI_OUTPUT (Shashank)

Cc: stable@vger.kernel.org
Cc: Tore Anderson <tore@fud.no>
Reported-by: Tore Anderson <tore@fud.no>
Fixes: 7a0baa62 ("Revert "drm/i915: Disable 12bpc hdmi for now"")
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Shashank Sharma <shashank.sharma@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1462362322-31278-1-git-send-email-ville.syrjala@linux.intel.comReviewed-by: Shashank Sharma <shashank.sharma@intel.com>