drm/amd/display: Remove unnecessary conversion to bool

Fix the following coccicheck warnings:

./drivers/gpu/drm/amd/display/dc/dcn30/dcn30_dpp_cm.c:298:33-38:
WARNING: conversion to bool not needed here.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>