i965: Only set Last Render Target Select on the last FB write.

i965: Only set Last Render Target Select on the last FB write.

Fixes GPU hangs in OilRush, Trine, and Amnesia: The Dark Descent,
which all use MRT (multiple render targets).

NOTE: This is a candidate for release branches.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38720
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=40059
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=45216
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>