msm: kgsl: flush the entire cache when the bulk batch is large

On 8064 and 8974, flushing more than 16mb of virtual address
space is slower than flushing the entire cache. So flush
the entire cache when the working set is larger than this.
The threshold for full cache flush can be tuned at runtime via
the full_cache_threshold sysfs file.

Change-Id: If525e4c44eb043d0afc3fe42d7ef2c7de0ba2106
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>