i965/fs: Set smear on shader_time diff register.

i965/fs: Set smear on shader_time diff register.

The ADD(diff, diff, fs_reg(-2u)) instruction reads diff, which is a
width 1 register.  We need to read it as <0,1,0> with a subreg of 0,
which is what smear accomplishes.

Fixes assertion:
brw_eu_emit.c:285: validate_reg: Assertion `hstride == 0' failed.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=86974
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Cc: mesa-stable@lists.freedesktop.org