Clear out the vGPU platform keys unconditionally

We need to do this even if there is no virtual GPU - we may have moved
from vGPU to PCI passthrough, or removed the vGPU altogether.

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #1430 from johnelse/clear-platform-flags

Clear out the vGPU platform keys unconditionally