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

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: eb5c10cbbc2f ("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