msm: kgsl: Fix context refcounting for adreno debugfs

The code is currently using a device ID look up for the context ID. This
is presumably because there was a refcounting race when looking at the
the debugfs file while the context was being destroyed.  Instead of
trying to be clever, just take a reference count on the context when the
debugfs file is created and release it when the context is detached. This
may cause the context to stick around a bit longer, but it sure is cleaner.

Change-Id: Ic0dedbad3f30168e74d03eade59218f82b93bee4
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>