drm/i915/guc: fix NULL vs IS_ERR() checking

The intel_engine_create_virtual() function does not return NULL.  It
returns error pointers.

Fixes: e5e32171 ("drm/i915/guc: Connect UAPI to GuC multi-lrc interface")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211116114916.GB11936@kili