i965: Fix Haswell discard regressions since Gen4-5 line AA fix.

i965: Fix Haswell discard regressions since Gen4-5 line AA fix.

In commit dc2d3a7f5c217a7cee92380fbf503924a9591bea, Iago accidentally
moved fire_fb_write() above the brw_pop_insn_state(), which caused the
SEND to lose its predication and change from WE_normal to WE_all.
Haswell uses predicated SENDs for discards, so this broke Piglit's
tests for discards.

We want the Gen4-5 MOV to be uncompressed, unpredicated, and unmasked,
but the actual FB write itself should respect those.  So, pop state
first, and force it again around the single MOV.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=79903