Fix GraphicsMapperStableCTests#LockUnlockNoCPUUsage

Add CPU usage into buffer descriptions for 2 reasons:
- minigbm doesn't support allocating buffers with usage == 0
- it could happen that the allocation is not cpu-mappable in the fist
  place and this woiuld make  the test to pass by accident

Bug: 374004471
Test: http://ab/I68300010330229584
Change-Id: I589e7673236107b92d3d6a2587eab55f81b8520d