compositor-drm: Extract drm_fb_addfb into a helper

We currently do the same thing in two places, and will soon have a
third.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Tested-by: Emre Ucan <eucan@de.adit-jv.com>