drm/amdgpu/vcn: fix allocation size in enc ring test

We need to allocate a large enough buffer for the
session info, otherwise the IB test can overwrite
other memory.

- Session info is 128K according to mesa
- Use the same session info for create and destroy

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=204241


Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: James Zhu <James.Zhu@amd.com>
Tested-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>