sf: vds: Allocate display ID for displays composed/copied by HWC

We make VDS responsible for deciding if a display will be composed
by HWC. This is primarily based on the usage flags on the sink
buffer, as well as on the requested display resolution.

With this approach, there are two possible scenarios in which we
make a decision that applies while the display is connected:

1. The display will be composed by HWC, and we use WB to produce
   the final output.
2. The display will be composed by GLES/GPU and HWC has no knowledge
   of this display.

For case 1) we will have a valid display ID and for case 2) we will
have NO_MEMORY as the display ID.

Change-Id: I92b1e8b5973fea1a894caa7588dd95a1e551dd8e