i965: Fix the response len of masked sampler messages for 8-wide dispatch.

i965: Fix the response len of masked sampler messages for 8-wide dispatch.

The bad response length would hang the GPU with a masked sample in a
shader using control flow.  For 8-wide, the response length is always
4, and masked slots are just not written to.  brw_wm_glsl.c already
allocates registers in the right locations.

Fixes piglit glsl-fs-bug25902 (fd.o bug #25902).
(cherry picked from commit f6d210c284751ac50a8d6358de7e75a1ff1e4ac7)
(cherry picked from commit dc8c0359448cdae7b367552ba58783c04b199778)