video: msm: Add logical display id for post processing

Define logical id for display backend processing.
These ids are independent with hw implementation, could be used
for MDP4, MDP5 and future ASICs. Kernel driver implementation will
also be abstracted to the post processing user.

Signed-off-by: Ken Zhang <kenz@codeaurora.org>
(cherry picked from commit 6a431631)

Change-Id: I27bd4e7d3803f4b356d924c65ac28e142375d1d5
Signed-off-by: Sudhir Sharma <sudsha@codeaurora.org>