drm/msm/dpu: extract bandwidth aggregation function

In preparation to refactoring the dpu_core_perf debugfs interface,
extract the bandwidth aggregation function from
_dpu_core_perf_crtc_update_bus().

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/636058/
Link: https://lore.kernel.org/r/20250209-dpu-perf-rework-v5-1-87e936cf3004@linaro.org