drm/i915/selftests: Use gt_err for GT info

It will be more informative regarding
GT if we use gt_err instead.

Cc: Andi Shyti <andi.shyti@intel.com>
Signed-off-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230428125952.4037964-3-tejas.upadhyay@intel.com