i965/gen8: Fix antialiased line rendering with width < 1.5

i965/gen8: Fix antialiased line rendering with width < 1.5

The same fix Marius implemented for gen6 (commit a9b04d8a) and
gen7 (commit 24ecf37a).

Also, we need the same code to handle special cases of line width
in gen6, gen7 and now gen8, so put that in the helper function
we use to compute the line width.

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