Writeback was the last user of dpu_encoder_phys_ops's atomic_check()
callback. As the code was moved to the dpu_writeback.c, the callback
becomes unused. Drop it now.

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/577528/
Link: https://lore.kernel.org/r/20240208-fd_remove_phys_ops_atomic_mode_set-v4-5-caf5dcd125c0@linaro.org