tests/amdgpu: annotate (most of) the vce templates as const data

The remaining two templates are modified on the fly, depending on the
type of test to be performed.

Cc: Leo Liu <leo.liu@amd.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>