drm/vmwgfx: Skipping fbdev fb pinning for ldu

Pinning fbdev's FB at the start of VRAM prevents X from pinning
its FB.  Since for ldu, the fb would be pinned anyway during a
mode set, just skip pinning it in fbdev.

This is not the best solution, but since ldu is not used much
anymore, it seems like a reasonable workaround.

Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>