drm/i915: add wrappers to get intel connector state

Wrap drm_atomic_get_old_connector_state so we can get the
intel_digital_connector_state and make it easier to migrate to intel
types.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191030012448.14937-3-lucas.demarchi@intel.com