Add a couple of simple store dword tests to test memory coherence.
gem_storedw_loop simply executes a batch that continually stores an
incremented value to a target buffer object, checking the results after
each batch completes.
gem_storedw_batches_loop does the same thing, but creates a new command
batch buffer for each iteration, which can exercise the buffer creation
code. This test is based on one from Andrzej Kacprowski from Intel.
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
