compositor-drm: factor out drm_output_init_crtc()

compositor-drm: factor out drm_output_init_crtc()

Factor out drm_output_init_crtc() and drm_output_fini_crtc(), so that
the call sites can later be moved easily.

On fini, reset scanout_plane and cursor_plane to NULL, so that in the
future when the drm_output is not longer destroyed immediately after, we
free the planes for other use.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Ian Ray <ian.ray@ge.com>
[Pekka: set crtc_id/pipe at top, reset both on error]
Reviewed-by: Daniel Stone <daniels@collabora.com>