Release outstanding suspend blockers when shutting down DPC

Right now DPC removes all messages but doesn't actually clean up all of
the outstanding state that's in flight. In general, this isn't that
harmful for most things as the display is going away at this point, but
if any suspend blockers are being held pending this work then we end up
in a state where the device can never suspend. This causes severe
battery drain that can only be fixed by a reboot.

Test: manual
Fixes: 213407479
Change-Id: I5b61f5bd498f09026d5b398bb3a8ccdaa91601fe