clear_pending_flush() ctl op is always assigned irrespective of the DPU
hardware revision. Hence there is no needed to check whether the op has
been assigned before calling it.

Drop the checks across the driver for clear_pending_flush() and also
update its documentation that it is always expected to be assigned.

changes in v2:
	- instead of adding more validity checks just drop the one for clear_pending_flush
	- update the documentation for clear_pending_flush() ctl op
	- update the commit text reflecting these changes

changes in v3:
	- simplify the documentation of clear_pending_flush

Fixes: d7d0e73f ("drm/msm/dpu: introduce the dpu_encoder_phys_* for writeback")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/all/464fbd84-0d1c-43c3-a40b-31656ac06456@moroto.mountain/T/Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/600241/
Link: https://lore.kernel.org/r/20240620201731.3694593-1-quic_abhinavk@quicinc.comSigned-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>