drm/amdgpu:update kernel vcn ring test

add session context buffer to decoder ring test.

v5 - clear the session ct buffer (Christian)
v4 - data type, explain change of ib size change (Christian)
v3 - indent and  v2 changes correction. (Christian)
v2 - put the buffer at the end of the IB (Christian)

Signed-off-by: Saleemkhan Jamadar <saleemkhan.jamadar@amd.com>
Acked-by: Leo Liu <leo.liu@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>