msm: mdss: add handoff support for multiple pipes on same mixer stage

In the existing implementation, display driver assumes only one pipe
can be staged on a layer mixer during overlay handoff. Starting from
MDSS v1.03, layer mixer can support two pipes staged at the same stage.
This change handles the case for two pipes on layer mixer at same
staging level. If more than two pipes are staged on a layer mixer, it
is invalid.

Change-Id: Ie6fa42e7a5b555de53fc4a3801e89447c3b1f90f
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>