msm: kgsl: Make sure that pool pages don't have any extra references

Before putting a page back in the pool be sure that it doesn't have
any additional references that would be a signal that somebody else
is looking at the page and that it would be a bad idea to keep it
around and run the risk of accidentally handing it to a different
process.

Change-Id: Ic0dedbad0cf2ffb34b76ad23e393c5a911114b82
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>