sf: Clean up properties for virtual display

Currently, the properties are used as follows:
1. debug.sf.enable_hwc_vds - allows IDs to be generated for VDs
2. vendor.display.vds_allow_hwc - allows WFD to use HWC path

With this change, HWC path is enabled for WFD with only the vendor
property set. All other virtual displays require both of the
properties enabled.

Change-Id: Iab2c8d15d2c1cf24be0d371af8892c346634507f
CRs-Fixed: 3204941