Add GPUVM size and DRM render node. Also add function to query the
VMID mask to avoid hard-coding it in multiple places later.

v2: cut off GPUVM size at the VA hole
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>