drm/msm/dpu: rename average bandwidth-related debugfs files

Rename the debugfs files to match their purpose and the patter provided
by other bandwidth and clock-related files:

threshold_high -> max_core_ab
threshold_low -> low_core_ab

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/636069/
Link: https://lore.kernel.org/r/20250209-dpu-perf-rework-v5-6-87e936cf3004@linaro.org