drm/amdgpu/discovery: validate VCN and SDMA instances

Validate the VCN and SDMA instances against the driver
structure sizes to make sure we don't get into a
situation where the firmware reports more instances than
the driver supports.

Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>