drm/msm/dpu: move writeback's atomic_check to dpu_writeback.c

dpu_encoder_phys_wb is the only user of encoder's atomic_check callback.
Move corresponding checks to drm_writeback_connector's implementation
and drop the dpu_encoder_phys_wb_atomic_check() function.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Tested-by: Paloma Arellano <quic_parellan@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/577524/
Link: https://lore.kernel.org/r/20240208-fd_remove_phys_ops_atomic_mode_set-v4-4-caf5dcd125c0@linaro.org