drm/msm/dpu: simplify dpu_encoder_get_topology() interface

As a preparation for calling dpu_encoder_get_topology() from different
code paths, simplify its calling interface, obtaining some data pointers
internally instead passing them via arguments.

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/633396/
Link: https://lore.kernel.org/r/20250123-drm-dirty-modeset-v2-3-bbfd3a6cd1a4@linaro.org