drm/msm/dpu: use DEFINE_SHOW_ATTRIBUTE to simplify dpu_regset32

Use DEFINE_SHOW_ATTRIBUTE helper macro to simplify the code.
No functional change.

Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/504252/
Link: https://lore.kernel.org/r/20220922142147.3246649-1-liushixin2@huawei.com


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>