hwc2: Cache display type at construction

The display type is constant for the life of the display. Query it
upfront to avoid the binder transaction later.

V2: fixup HWC1 path

Test: boot to launcher on bullhead; build on fugu for HWC1 path
Bug: b/36597125
Change-Id: I2a4c3d9ff449960957376afef78f424261fcc282