PM: core: Tweak pm_runtime_block_if_disabled() return value

Modify pm_runtime_block_if_disabled() to return true when runtime PM
is disabled for the device, regardless of the power.last_status value.

This effectively prevents "smart suspend" from being enabled for
devices with runtime PM disabled in device_prepare(), even transiently,
so update the related comment in that function accordingly.

If a device has runtime PM disabled in device_prepare(), it is not
actually known whether or not runtime PM will be enabled for that
device going forward, so it is more appropriate to postpone the
"smart suspend" optimization for the device in the given system
suspend-resume cycle than to enable it and get confused going
forward.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://patch.msgid.link/13718674.uLZWGnKmhe@rjwysocki.net