hwc: Partial update support for ping pong split.

ping-pong split needs source split to be enabled to stage two
pipes of a layer in the same Z order. This change adds partial
update support to generate ROI when source split is enabled.
When configured, ping-pong-split splits the layer mixer output equally
across two DSI's. To meet this limitation, the generated ROI is
adjusted to have equal widths from the panel mid point.

Change-Id: Ic307a1d5b902af9e352b0dfc6f5ab474ecd9c8a4
4 files changed