compositor: Re-work the DRM and other backends loading phase

This follows Weston frontend code for loading up and head/output
enablement. RDP, Wayland and X11 backend loading are all handled
through the same common code, while the DRM one has a clear distinct
one. This greatly simplifies the loading phase and provides a more
easier, intuitive code review.

Bug-AGL: SPEC-5096, SPEC-5061
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I97d39004dea0e030478f27ea18989887b8e2cc82