drm/i915/ddi: set has_infoframe flag on DDI too v2

Just like we do in the HDMI code, set the infoframe flag if we detect
that infoframes are enabled.

v2: check for actual infoframe status as in hdmi code (Daniel)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>