drm: introduce drm_writeback_connector_init_with_encoder() API

For vendors drivers which pass an already allocated and
initialized encoder especially for cases where the encoder
hardware is shared OR the writeback encoder shares the resources
with the rest of the display pipeline introduce a new API,
drm_writeback_connector_init_with_encoder() which expects
an initialized encoder as a parameter and only sets up the
writeback connector.

changes in v5:
	- fix the encoder doc to indicate that its not valid for
	  users of drm_writeback_connector_init_with_encoder()

Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Patchwork: https://patchwork.freedesktop.org/patch/483500/
Link: https://lore.kernel.org/r/1650984096-9964-3-git-send-email-quic_abhinavk@quicinc.com


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>