i965: Don't forget the force_sechalf flag in lower_load_payload().

Regression from commit 41868bb682.
Fixes a bunch of ARB_shader_image_load_store tests.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -3507,6 +3507,7 @@ fs_visitor::lower_load_payload()
             fs_inst *mov = MOV(retype(dst, inst->src[i].type),
                                inst->src[i]);
             mov->force_writemask_all = inst->force_writemask_all;
+            mov->force_sechalf = inst->force_sechalf;
             inst->insert_before(block, mov);
          }
          dst = offset(dst, 1);