disp: msm: sde: add cwb virtual mixers support

In yupik target, cwb path has pingpong block without any layer mixer.
Add virtual mixer to support reservation of pp block in such case.
This virtual mixer will be used only for cwb use case.

Change-Id: Iacc1a3f5e7d99d9fc36288450b0e19081b3ef763
Signed-off-by: Andhavarapu Karthik <kartkart@codeaurora.org>