drm/i915/ddi: Check transcoder instead of port when setting HDMI infoframe

The only requirement by BSpec for setting the HDMI infoframes is on DDI
platforms to do that before enabling the HDMI transcoder function, see
VIDEO_DIP_CTL bit 16. Accordingly check for the transcoder function
disabled state instead of the port's disabled state on DDI platforms.
This is needed by the next patch as it will set the infoframe during
crtc disabling where the port is still enabled.

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180613170710.15080-4-imre.deak@intel.com