ANDROID: GKI: Unhide VIRTIO_DMA_SHARED_BUFFER

So downstream boards can select it. It is needed by some virtio drivers
like virtio-gpu. We can't put this in init/Kconfig.gki because we don't
want to enable VIRTIO options in GKI if they can be tristate.

[CPNOTE: 21/07/21] Lee: Asked the author for status

Bug: 175707513
Change-Id: Ic8f7b0aa0ed6e9c74cda08d96eb30507d6886275
Signed-off-by: Alistair Delva <adelva@google.com>
Signed-off-by: Lee Jones <joneslee@google.com>