drm/i915: fix ILK+ infoframe support

Misc fixes based on tests with an infoframe analyzer:
  - checksum *does* include header bytes
  - DIP enable & AVI infoframe are tied together in hw, so disable both
    and make sure AVI frames are enabled first
  - use every vsync flag for SPD frames to avoid reserved value in
    frequency field when enabling both AVI & SPD

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=40281

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Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: stable@kernel.org
Signed-off-by: Keith Packard <keithp@keithp.com>