drm/msm/dpu: move needs_cdm setting to dpu_encoder_get_topology()

As a preparation for calling dpu_encoder_get_topology() from different
places, move the code setting topology->needs_cdm to that function
(instead of patching topology separately).

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>