gralloc: Allow legacy devices to use PMEM for ADSP (camera).

Partially bring back PMEM support for legacy devices having proprietary camera
libraries stuck in PMEM. This change allows these devices to continue using
PMEM for the camera, and ION for everything else.

Change-Id: I1382381134db21435ca725d0e2c2b3194da16f38
gralloc: Let gralloc_priv.h be overloaded by device trees

Adjustments to private_handle_t may be necessary for device-specific
blob compatibility, so don't enforce usage of the local header

Change-Id: I588ca59ed76b1a063a4b663ac2c98589865b7738