The function drm_connector_assign_connector_info() should
not be calling functions to handle drm_head, as connectors
and heads are not the same thing after patch "drm-backend:
move connector data from struct drm_head to struct drm_connector".
Move drm_head specific calls to drm_head_update_info(). This
is more in line with the hierarchy of the objects and also
allow us to drop drm_head pointer from drm_connector.
Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>