mdss: mdp: Create virtual wb mixers for rotation if not available

Create virtual wb mixers to support rotation, if no matching
mixers are available in the model. The number of virtual mixers
is equal to the number of DMA pipes available for rotation.

Change-Id: I3f4df3175595168d55373eb91e5cd05ef5f65919
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>