i965: Move some code from gen7_sol_state.c to gen6_sol.c.

i965: Move some code from gen7_sol_state.c to gen6_sol.c.

I plan to use these functions on Sandybridge soon.  I changed the prefix
on a couple of functions to "brw" instead of "gen7" as in theory they
should be usable all the way back to G45.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>