i965: Combine brw_emit_prim and gen7_emit_prim.

i965: Combine brw_emit_prim and gen7_emit_prim.

These functions have almost identical code; the only difference is that
a few of the bits moved around.  Adding a few trivial conditionals
allows the same function to work on all generations, and the resulting
code is still quite readable.

v2: Comment that the workaround flush is only necessary on SNB
    (requested by Paul Berry).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Paul Berry <stereotype441@gmail.com>