drm/gma500: Make use of the drm connector iterator

This makes sure we're using proper locking when iterating the list of
connectors.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220322131742.11566-3-patrik.r.jakobsson@gmail.com