drm/i915: Track hdmi mode in the pipe config

Also add state readout and cross-check support. The only invasive change
is wiring up the new flag to the ->set_infoframes callbacks.

Reviewed-by: Naresh Kumar Kachhi <naresh.kumar.kachhi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>