ASoC: Disable WM8994 VMID for digital only paths

On WM8994 class devices only the analogue portions of the CODEC require
VMID so when running digital only paths we can leave VMID disabled.
On some earlier devices the FLL uses VMID so we don't use DAPM reference
counting alone, we maintain an internal reference count which is also
enabled and disabled by the FLL startup.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>