hwc: Add API to find if a display is split

A display could be split if its dimensions exceed 2048 or in case
of primary if we make it so via device tree property.

Add an API to find if a display is split. Use this API instead of
going through a set of conditions in various use cases.
Rename the HighRes, LowRes objects to a more appropriate Split and
NonSplit respectively

Change-Id: Id847c2bf36ea9250e1cdbc0259fdec64bd124537
7 files changed