drm/amdgpu: fix possible UAF in amdgpu_cs_pass1()

Since the gang_size check is outside of chunk parsing
loop, we need to reset i before we free the chunk data.

Suggested by Ye Zhang (@VAR10CK) of Baidu Security.

Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org