i965: fix up asserts in brw_inst_set_jip()

We are casting from a signed 32bit int to an unsigned 16bit int
so shift 15 bits rather than 16.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/brw_inst.h

@@ -283,8 +283,8 @@ brw_inst_set_jip(const struct gen_device_info *devinfo,
    if (devinfo->gen >= 8) {
       brw_inst_set_bits(inst, 127, 96, (uint32_t)value);
    } else {
-      assert(value <= (1 << 16) - 1);
-      assert(value > -(1 << 16));
+      assert(value <= (1 << 15) - 1);
+      assert(value >= -(1 << 15));
       brw_inst_set_bits(inst, 111, 96, (uint16_t)value);
    }
 }