i965: Fix writemask != 0 assertions on Sandybridge.

i965: Fix writemask != 0 assertions on Sandybridge.

This fixes myriads of regressions since commit 169f9c030c16d1247a3a7629
("i965: Add an assertion that writemask != NULL for non-ARFs.").

On Sandybridge, our control flow handling (such as brw_IF) does:

   brw_set_dest(p, insn, brw_imm_w(0));
   insn->bits1.branch_gen6.jump_count = 0;

This results in a IMM destination with zero for the writemask.  IMM
destinations are rather bizarre, but the code has been working for ages,
so I'm loathe to change it.

Fixes glxgears on Sandybridge.

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