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

Everybody loves a rcu but in this case we are dangerously mixing rcu's
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: Ic2f1e5c45692683b52a9f662ccdd2fb59bc7ae0e
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Harsh Vardhan Dwivedi <hdwivedi@codeaurora.org>