drm/amdgpu: get VCN and SDMA instances from IP discovery table

Rather than hardcoding it.  We already have the number of VCN
instances from a previous patch, so just update the VCN
instances for chips with static tables.

v2: squash in checks for SDMA3,4 (Guchun)
v3: clarify VCN changes

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