Comment 3 for bug 1460868

It is intentional. C.f. -c 2295 and -c 2316. From the former commit message:

  Drop libmirclient8 dependency on client-platform drivers.

  Similar to libX11, which doesn't depend on an X11 server.

  This dependency is satisfied by other parts of the stack; the desktop-next and touch
  seeds.

This implies that the seed ought to be pulling in the drivers.

I don't have a wily install to hand but it sounds like mir-graphics-drivers-desktop is still pulling mir-platform-graphics-mesa1. (Which is what it does on vivid.)