i965/fs: Do a general SEND dependency workaround for the original 965.

i965/fs: Do a general SEND dependency workaround for the original 965.

We'd been ad-hoc inserting instructions in some SEND messages with no
knowledge of when it was required (so extra instructions), but not all SENDs
(so not often enough).  This should do much better than that, though it's
still flow-control-ignorant.

v2: Use BRW_MAX_MRF instead of magic numbers.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=58960
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
NOTE: Candidate for the stable branches.