drm/i915/hdmi: Port the infoframe code to the common hdmi helpers

Let's use the drivers/video/hmdi.c and drm infoframe helpers to build
our infoframes.

v2: Simplify the logic to compute the buffer size. We can just take the
maximum infoframe size rounded to 32, which happens to be what the
hardware let us write anyway.

v3: Remove unnecessary memset() (Ville Syrjälä)

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>