drm/amdgpu: Use discovery table's subrevision

Use subrevision of IP version in discovery table to identify SOC
revision id for NBIO v7.9 SOCs. Only newer bootloaders update
subrevision field.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Le Ma <le.ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>