UPSTREAM: PM: Avoid false-positive warnings in dev_pm_domain_set()

There is a WARN_ON() in dev_pm_domain_set() that triggers on attempts
to set the pm_domain pointer for devices with a driver bound.

However, that WARN_ON() triggers on attempts to clear the pointer
too and the test it uses is based on checking the device's
p->knode_driver pointer which still is set when the device bus
type's/driver's ->remove callback has been executed.  This
leads to false-positive warnings when bus type code calls
dev_pm_domain_set() to clear the pm_domain pointer after
invoking the driver's ->remove() callback.

To avoid those false-positives, make dev_pm_domain_set() check
if the pointer passed to it is NULL and skip the warning in
that case.

Fixes: 989561de9b51 (PM / Domains: add setter for dev.pm_domain)
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Sinan Kaya <okaya@codeaurora.org>
Tested-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
(cherry picked from commit e79aee49bcf9ad005b3f2f4cf68b17dc2c22d9c2)

BUG=chromium:810200
TEST=Build and boot

Change-Id: I772ceae402b2568b8f596bf812422335c9404848
Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Reviewed-on: https://chromium-review.googlesource.com/907771
Commit-Ready: Brian Norris <briannorris@chromium.org>
Tested-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>