modetest: add and use bo_fb_create() helper

Flesh out the bo_create + drmModeAddFB2 dance into a helper and use it.
Currently we're duplicating that in 4 places, many of which leaking et
al.

As a bonus point this highlights that the atomic_set_plane() seems tad
buggy. That'll be fixed with separate commit.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
Tested-by: Ezequiel Garcia <ezequiel@collabora.com>