drm/msm/dpu: remove unused fields from struct dpu_core_perf

Remove dpu_core_perf::dev and dpu_core_perf::debugfs_root fields, they
are not used by the code.

Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/550200/
Link: https://lore.kernel.org/r/20230730010102.350713-8-dmitry.baryshkov@linaro.org