hwc: Don't permit rotator use under certain conditions

These include constrained targets, with external connected which could
lead to a scarcity of pipes. Furthermore owing to a scarcity of writeback
interfaces, even if pipes are available, we may not be able to still
use rotator.

Change-Id: Ie2fb4c1726f32addcace89c268a126abe0cddbe0
6 files changed