Limit #ROT-SESSIONS to 4 in LayerRotMap and RotMgr

* Limit the number of rotator sessions to 4 both in
LayerRotMap and RotMgr structures

* Make use of MDPVersion getter fuctions to check
for maxMixerWidth and maxDownScale supported by MDP

Change-Id: I27dac877807fbb4ed6fcb3500a3a318a2ef5d8b2