xf86drmMode: introduce drmModeConnectorGetPossibleCrtcs

Nowadays, users don't really care about encoders except for retrieving
the list of CRTCs compatible with a connector. Introduce a new function
so that users no longer need to deal with encoders.

This is a re-do of [1], but with a slightly different API.

Signed-off-by: Simon Ser <contact@emersion.fr>

[1]: https://gitlab.freedesktop.org/mesa/drm/-/merge_requests/102