virgl: Clear the valid buffer range when possible

virgl: Clear the valid buffer range when possible

If we are discarding the whole resource, we don't care about previous contents,
and the resource storage is now unused, either because we have created new
resource storage, or because we have waited for the existing resource storage
to become unused, or because the transfer is unsynchronized.

In the last two cases this commit marks the storage as uninitialized, but only
if the resource is not host writable (in which case we can't clear the valid
range, since that would result in missed readbacks in future transfers).

In the first case, when the whole resource discard involves a reallocation, the
reallocation and subsequent rebinding already update the valid buffer range
appropriately.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>