msm: kgsl: disable full cache flush if full_cache_threshold is 0

Make full_cache_threshold==0 mean never do a full cache flush,
since that is more useful than having it mean always do it.

Change-Id: I4ae46b6253f2d78a97f067b5f906637a3db1d5c8
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>