Use dev_from_gk20a() accessor whenever accessing struct device * from struct gk20a. JIRA NVGPU-38 Change-Id: Ide9fca3a56436c8f62e7872580a766c4c1e2353e Signed-off-by: Terje Bergstrom <tbergstrom@nvidia.com> Reviewed-on: https://git-master/r/1507930 Reviewed-by: Automatic_Commit_Validation_User GVS: Gerrit_Virtual_Submit