graphics: fix readback buffer tests

Make readback buffer support optional.  Fix incorrect use of
setReadbackBuffer and getReadbackBufferFence.

Bug:  80030364
Test: VTS
Change-Id: I278a031c3c086ac38c460b7076a2952db57a91a4
Merged-In: I278a031c3c086ac38c460b7076a2952db57a91a4
3 files changed