drm/i915/selftest: Record GT error for gt failure

igt_live_test has pr_err dumped in case of some
GT failures. 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/20230424133607.3736825-1-tejas.upadhyay@intel.com