drm/i915/selftests: Fix inconsistent IS_ERR and PTR_ERR

Fix inconsistent IS_ERR and PTR_ERR in i915_gem_dmabuf.c

Signed-off-by: Kai Song <songkai01@inspur.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211022120655.22173-1-songkai01@inspur.com