nvhost: Add locking to module clock code

nvhost: Add locking to module clock code

nvhost_module_add_client(), nvhost_module_remove_client() and
nvhost_module_set_rate() need locking when accessing the client list.

Bug 870328

(cherry picked from commit a09309d672fd2f96be1b61d58c57dcc659710e92)

Change-Id: I4c556df0e9607b3e2effb3b788220b784f858774
Reviewed-on: http://git-master/r/52845
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>
Tested-by: Terje Bergstrom <tbergstrom@nvidia.com>
Reviewed-by: Yu-Huan Hsu <yhsu@nvidia.com>
Reviewed-by: Scott Williams <scwilliams@nvidia.com>
Rebase-Id: R379d4317b22c1360a4756e28fbf66655e3aded95