gpu: nvgpu: gk20a: do not access invalid TLB

gpu: nvgpu: gk20a: do not access invalid TLB

Problem description:
 - gk20a_mm_tlb_invalidate guarantees callers that
   TLB would be in a clean state
 - gk20a_mm_tlb_invalidate checks whether the dirty
   flag is set, then mark clean, then performs
   reset operation
 - If there are two parallel callers, one thread would be
   performing the clean up operation, while the other thread
   is returned to the caller

Fix description:
 - Extends the duration of the TLB dirty flag mutex lock

Bug 1519537

Change-Id: Ib762d876ea11c06cc39399b1e2607ae3a2c0e17e
Signed-off-by: Sang-Hun Lee <sanlee@nvidia.com>
Reviewed-on: http://git-master/r/422820
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Shridhar Rasal <srasal@nvidia.com>
Reviewed-by: Mitch Luban <mluban@nvidia.com>