drm/i915/gvt: Use list_entry to access list members

Use list_entry() instead of container_of() to access list members.
Also drop unnecessary and misleading NULL checks on the result of
list_entry().

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20210523172304.3033229-1-linux@roeck-us.net


Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>