vc4: Add labels to BOs for debug builds or with VC4_DEBUG=surf set.

vc4: Add labels to BOs for debug builds or with VC4_DEBUG=surf set.

This has proven to be incredibly useful for debugging CMA allocation
failures and driving memory management improvements.  However, we don't
want to burden entry and exit from the BO cache with the labeling ioctl's
overhead on release builds.