i965: Handle IVB DF differences in the validator.

i965: Handle IVB DF differences in the validator.

On IVB/BYT, region parameters and execution size for DF are in terms of
32-bit elements, so they are doubled. For evaluating the validity of an
instruction, we halve them.

v2 (Sam):
- Add comments.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>