drm/i915: Remove "i2c_speed" nonsense from child device table

I have no evidence for this byte being used this way, and lots of
counterexamples.  Restore the struct to its empirical definition and
patch up gmbus setup to match.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Keith Packard <keithp@keithp.com>