drm/amdgpu: add new member in amdgpu_device for vmhub counts per asic chip

It aims to replace AMDGPU_MAX_VMHUBS in for loop to initialize registers.

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