Dequeuing and queuing the graphics buffer takes care of ownership, and
a previous patch ensured the buffer is unlocked before we queue it so
we don't need to hold a reference for longer.

Bug b/34981145
Bug b/35443034

Change-Id: I6cbaeff08f5d930dde8bdd1522052ac33a6ae9ac
Reviewed-on: https://swiftshader-review.googlesource.com/8849Reviewed-by: Greg Hartman <ghartman@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Greg Hartman <ghartman@google.com>