tests/amdgpu/vcn: fix session buffer issue for vcn1-vcn3

issue: in vcn1-vcn3, session buffer was not truly added, it shows decoding creation commands cannot be sent multiple times.

problem: session buffer has to be the first buffer sending out. Otherwise, system could assume session buffer doesn't exist.

solution: move session buffer sending sequence to be the first one.

Signed-off-by: Ruijing Dong ruijing.dong@amd.com