drm/amd/amdgpu: fix comparison pointer to bool warning in sdma_v4_0.c

Fixes coccicheck warning:

drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c:1003:4-9: WARNING: Comparison to bool
drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c:1083:5-11: 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>