i965: Factor out oword block read and write message control calculation.

i965: Factor out oword block read and write message control calculation.

We'll need roughly the same logic in other places and it would be
annoying to duplicate it.  Instead factor it out into a function-like
macro that takes the number of dwords per block (which will prove more
convenient than taking the same value in owords or some other unit).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>