drm/i915/gt: Use the correct error value when kernel_context() fails

kernel_context() returns an error pointer. Use pointer-error
conversion functions to evaluate its return value, rather than
checking for a '0' return.

Fixes: eb5c10cb ("drm/i915: Remove I915_USER_PRIORITY_SHIFT")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: <stable@vger.kernel.org> # v5.13+
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Acked-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230526124138.2006110-1-andi.shyti@linux.intel.com

drivers/gpu/drm/i915/gt/selftest_execlists.c

@@ -1530,8 +1530,8 @@ static int live_busywait_preempt(void *arg)
 	struct drm_i915_gem_object *obj;
 	struct i915_vma *vma;
 	enum intel_engine_id id;
-	int err = -ENOMEM;
 	u32 *map;
+	int err;
 
 	/*
 	 * Verify that even without HAS_LOGICAL_RING_PREEMPTION, we can
@@ -1539,13 +1539,17 @@ static int live_busywait_preempt(void *arg)
 	 */
 
 	ctx_hi = kernel_context(gt->i915, NULL);
-	if (!ctx_hi)
-		return -ENOMEM;
+	if (IS_ERR(ctx_hi))
+		return PTR_ERR(ctx_hi);
+
 	ctx_hi->sched.priority = I915_CONTEXT_MAX_USER_PRIORITY;
 
 	ctx_lo = kernel_context(gt->i915, NULL);
-	if (!ctx_lo)
+	if (IS_ERR(ctx_lo)) {
+		err = PTR_ERR(ctx_lo);
 		goto err_ctx_hi;
+	}
+
 	ctx_lo->sched.priority = I915_CONTEXT_MIN_USER_PRIORITY;
 
 	obj = i915_gem_object_create_internal(gt->i915, PAGE_SIZE);