drm/amdgpu: Remove unnecessary conversion to bool

Fix the following coccicheck warnings:

./drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c:2252:40-45: 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>