compositor-drm: move mode list to set_mode()

Move the initialization of the drm_output mode list to
drm_output_set_mode() time.

Once we stop creating the drm_head with the drm_output, there will not
be a head to get the mode list from at drm_output creation time.

Furthermore, once DRM-backend starts supporting more than one head per
output, the combined mode list to be exposed to clients (and the
compositor?) must be constructed with all heads attached.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>