drm/amdkfd: Don't take process mutex for svm ioctls

SVM ioctls take proper svms->lock to handle race conditions, don't need
take process mutex to serialize ioctls. This also fixes circular locking
warning:

WARNING: possible circular locking dependency detected

  Possible unsafe locking scenario:

        CPU0                    CPU1
        ----                    ----
   lock((work_completion)(&svms->deferred_list_work));
                                lock(&process->mutex);
                     lock((work_completion)(&svms->deferred_list_work));
   lock(&process->mutex);

   *** DEADLOCK ***

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>