i965/fs: Set LastRT on the final FB write on Broadwell.

i965/fs: Set LastRT on the final FB write on Broadwell.

In Piglit's EXT_framebuffer_multisample/alpha-to-coverage-dual-src-blend
test, key->nr_color_regions == 2, but the dual source blend FB write has
ir->target set to 0.  So we failed to set "Last Render Target Select" on
any FB write message.

We only emit one FB write per render target, so my comment about setting
LastRT on every FB write directed at the last color region is a bit...
misinformed.  According to the documentation, depth buffer writes and
scoreboard updates happen on the FB write with LastRT set, so I believe
we want to set it only once.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Cc: "10.2" <mesa-stable@lists.freedesktop.org>