drm/amdgpu/gfx12: recalculate available compute rings to use

Recalculate the number of compute rings to use based on
the gfx hardware configuration. As needed reserve half of
compute rings for mes, kgd can't use up all compute rings.

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>