intel/fs: grab fail_msg from v32 instead of v16 when v32->run_cs fails

Looks like a copy/paste error. This patch prevents a segfault when
running the following on BDW:

    INTEL_DEBUG=no8,no16,do32 ./deqp-vk -n \
        dEQP-VK.subgroups.arithmetic.compute.subgroupmin_dvec4

For the curious, the message we're getting is:

    CS compile failed: Failure to register allocate.  Reduce number
    of live scalar values to avoid this.

Fixes: 864737ce ("i965/fs: Build 32-wide compute shader when needed.")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
(cherry picked from commit 848d5e44)