gpu: nvgpu: gk20a: add busy lock

gpu: nvgpu: gk20a: add busy lock

- add rw_semaphore busy_lock for gpu busy() path
- take read lock on busy_lock inside gk20a_busy()
  so that all usual requests can execute simultaneously
- write lock can be taken when we need to block all
  of the gk20a_busy() calls

Bug 1487804

Change-Id: I1b162b38bce9621723d3e45280c6076816cf771a
Signed-off-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-on: http://git-master/r/412060
(cherry picked from commit 725f970aa378619bc9f0a928cd22fdaaf42698e5)
Reviewed-on: http://git-master/r/421695
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>
GVS: Gerrit_Virtual_Submit