drm/i915: Disable infoframes when shutting down DDI HDMI

Disabling the video DIP when shutting the port down seems like a good
idea.

Bspec says:
"When disabling both the DIP port and DIP transmission,
 first disable the port and then disable DIP."
and
"Restriction : GCP is only supported with HDMI when the bits per color is
 not equal to 8. GCP must be enabled prior to enabling TRANS_DDI_FUNC_CTL
 for HDMI with bits per color not equal to 8 and disabled after disabling
 TRANS_DDI_FUNC_CTL"

So let's do it in the .post_disable() hook.

v2: Remove double "dpms off" caused by rebase fail

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170822140914.24413-1-ville.syrjala@linux.intel.com