drm/komeda: Convert to DEFINE_SHOW_ATTRIBUTE

Use DEFINE_SHOW_ATTRIBUTE macro to simplify the code.

Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Tested-by: Carsten Haitzler <carsten.haitzler@foss.arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200917123949.101925-1-miaoqinglang@huawei.com