drm/amd/amdgpu: fix comparison pointer to bool warning in si.c

Fixes coccicheck warning:

drivers/gpu/drm/amd/amdgpu/si.c:1342:5-10: WARNING: Comparison to bool

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>