drm/amdgpu: Initialize timestamp for some legacy SOCs

Initialize the interrupt timestamp for some legacy SOCs
to fix the coverity issue "Uninitialized scalar variable"

Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Suggested-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>