drm/amdgpu/discovery: add ih6 support for IH 6.0.2

Enable Interrupt Handler v6 support.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Flora Cui <flora.cui@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>