gpu: nvgpu: prepare_poweroff() in shutdown()

gk20a_pm_shutdown() is the last callback before
GPU railgate will be forced by platform code

Hence we need to call prepare_poweroff() before
returning from shutdown() to clean up below things
mainly,

1. disable interrupts to ensure that GPU is not
   processing any interrupts while railgating

2. disable clocks (and related flags) to ensure
   no h/w access from exported clock ops

Note that GPU railgate will be triggered by platform
code since config CONFIG_PM_GENERIC_DOMAINS_OF is
enabled by default

Bug 200123584

Change-Id: Ifaa0d1ba9b01d49bf5cc85d9c9a9feb3815866d8
Signed-off-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-on: http://git-master/r/770485
Reviewed-by: Prashant Gaikwad <pgaikwad@nvidia.com>
Reviewed-by: Sachin Nikam <snikam@nvidia.com>

drivers/gpu/nvgpu/gk20a/gk20a.c

@@ -1148,6 +1148,9 @@ static void gk20a_pm_shutdown(struct platform_device *pdev)
 			break;
 	}
 #endif
+
+	/* Be ready for rail-gate after this point */
+	gk20a_pm_prepare_poweroff(&pdev->dev);
 }
 
 #ifdef CONFIG_PM