drm/amdgpu: Set the default value about gds vmid0 size

For sriov, when first run windows guest, then run linux guest, the gds
vmid0 size will be reset to 0 by windows guest. So if the value has been
reset to 0, then set the value to the default value in linux guest.

v2:
Fixed value instead of reading mmGDS_VMID0_SIZE.

v3:
Set the default value of the switch.

Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>