drm/vmwgfx: Treat out-of-range initial width and height as host errors

And assign the initial width and height to the minimum in that case.
Strange values (-1) from these registers have been reported by users.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>