drm/i915/guc/slpc: Use i915_probe_error instead of drm_err

This will ensure we don't have false positives when we run
error injection tests.

Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220412224852.21501-1-vinay.belgaumkar@intel.com