drm/gma500: remove an unneeded NULL check

"connector" is the list iterator and it can't be NULL.  It causes a
static checker warning because we dereference the iterator to get the
next item in the list.  Let's remove this check.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170628124100.3pw2gyitsfopaib5@mwanda