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

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

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