vmwgfx: Make vmw_wait_seqno a bit more readable

Break out on-demand enabling and disabling of fence irqs to make
the function more readable. Also make dev_priv->fence_queue_waiters an int
instead of an atomic_t since we only manipulate it with dev_priv->hw_mutex
held.

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