msm: kgsl: Use a read/lock for the context idr

Everybody loves a rcu but in this case we are dangerously mixing rcus and
atomic operations.  Add a read/write lock to explicitly protect the idr.
Also fix a few spots where the idr was used without protection.

Change-Id: Ic0dedbad517a9f89134cbcf7af29c8bf0f034708
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>