drm/vmwgfx: Use the new interface for SOU plane update

With new interface to do plane update on SOU available, use that instead
of old kms_dirty.

Signed-off-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>