hwc2: Remove Display::{mIsVirtual,setVirtual}

Trivial cleanup.

Now that mType is available, we don't need a separate flag for whether
the display is virtual. The remaining effect of setVirtual was to set
the display connected, so just use setConnected.

Bug: b/36597125
Test: build
Change-Id: Ia1fa4a324cc07b0d6c124f4f50966c8b0cbe4827