drm/vc4: fix itnull.cocci warnings

Connector cannot be null because it is a list entry, ie accessed at an
offset from the positions of the list structure pointers themselves.

Generated by: scripts/coccinelle/iterators/itnull.cocci

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Eric Anholt <eric@anholt.net>