virgl: hide fence internals from the driver

virgl: hide fence internals from the driver

Fence fds are cheaper than resources.  We want to let winsys make the
decision and use fence fds whenever they are supported.  This commit
prepares the work.

For the moment, we create a resource _and_ a fence fd when
supports_fences is true.  This will be fixed such that we create a
resource _or_ a fence fd.  (And because of a version check bug that we
will fix later, supports_fences is actually never true).

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>