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.

Reference (and many thanks): Blefish/android_hardware_qcom_display-caf/@41c51319b03

Change-Id: I0a1101c410f5d7b73dc3f16e5ffefb5ec5708891