drm/komeda: Accept null writeback configurations for writeback

User may send null writeback configurations for writeback connector like:
- Only bind the writeback connector to crtc.
- set a fb_id(0) to writeback_fb_id_property
All above configurations are meaningless for writeback, but since they are
still valid configurations, accept them.

Depends on:
- https://patchwork.freedesktop.org/series/60856/

Signed-off-by: James Qian Wang (Arm Technology China) <james.qian.wang@arm.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>