freedreno/ir3: fix new gcc6 errors

freedreno/ir3: fix new gcc6 errors

src/gallium/drivers/freedreno/ir3/ir3_compiler_nir.c: In function ‘emit_tex’:
src/gallium/drivers/freedreno/ir3/ir3_compiler_nir.c:1368:26: warning: unused variable ‘const_off’ [-Wunused-variable]
  struct ir3_instruction *const_off[4];
                          ^~~~~~~~~
unused since:

commit 8750299a420af76cebd3067f6f603eacde06ae06
Author: Jason Ekstrand <jason.ekstrand@intel.com>
Date:   Tue Feb 9 14:51:28 2016 -0800

    nir: Remove the const_offset from nir_tex_instr

Signed-off-by: Rob Clark <robdclark@gmail.com>