compositor-drm: introduce drm_head_read_current_setup()

Rename connector_get_current_mode() because it will be useful for
storing not just the current mode on creating a head.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derekf@osg.samsung.com>