intel/compiler: Silence maybe-uninitialized warning in GCC 9.1.1

intel/compiler: Silence maybe-uninitialized warning in GCC 9.1.1

Compiler can't see that d is initialized.

    ../src/intel/compiler/brw_vec4_nir.cpp: In function ‘int brw::try_immediate_source(const nir_alu_instr*, brw::src_reg*, bool, const gen_device_info*)’:
    ../src/intel/compiler/brw_vec4_nir.cpp:984:12: warning: ‘d’ may be used uninitialized in this function [-Wmaybe-uninitialized]
      984 |          d = MAX2(-d, d);

Assert that we expect at least one component -- hence d going to be
set.  That by itself is not enough, so also zero initialize the
variable.

Acked-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>