drm/vmwgfx: Use new validation interface for the modesetting code v2

Strip the old KMS helpers and use the new validation interface also in
the modesetting code.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com> #v1
Reviewed-by: Sinclair Yeh <syeh@vmware.com>