drm/amdgpu: correct chunk_ptr to a pointer to chunk.

The variable "chunk_ptr" should be a pointer pointing
to a struct drm_amdgpu_cs_chunk instead of to a pointer
of that.

Signed-off-by: YuanShang <YuanShang.Mao@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>