drm/amdgpu: add bitmask to iterate vmhubs

As the layout of VMHUB definition has been changed to cover multiple
XCD/AID case, the original num_vmhubs is not appropriate to do vmhub
iteration any more.

Drop num_vmhubs and introduce vmhubs_mask instead.

v2: switch to the new VMHUB layout
v3: use DECLARE_BITMAP to define vmhubs_mask

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