i965/fs: Ignore compute shaders in brw_nir_lower_inputs

i965/fs: Ignore compute shaders in brw_nir_lower_inputs

The commit shown below caused compute shaders to hit the unreachable
in the default of the switch block. Since compute shaders don't have
any inputs, we can make brw_nir_lower_inputs a no-op for CS.

commit 2953c3d76178d7589947e6ea1dbd902b7b02b3d4
Author: Kenneth Graunke <kenneth@whitecape.org>
Date:   Fri Aug 14 15:15:11 2015 -0700

    i965/vs: Map scalar VS input locations properly; avoid tons of MOVs.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>