Adjust fields of struct drm_gem_object for uvm.

Add a struct uvm_object pointer gemo_uao for an anonymous object to
back shared memory, and a struct uvm_object gemo_uvmobj for custom
device page mapping.

In struct drm_driver, replace the Linux VM operations gem_vm_ops by
NetBSD UVM operations gem_uvm_ops, so that the compiler will catch
any extant references to the Linux structure that will need changing.

(riastradh)