drm/i915: Flush GT idle status upon reset

Upon resetting the GPU, we force the engines to be idle by clearing
their request lists. However, I neglected to clear the GT active status
and so the next request following the reset was not marking the device
as busy again. (We had to wait until any outstanding retire worker
finally ran and cleared the active status.)

Fixes: 67d97da3

 ("drm/i915: Only start retire worker when idle")
Testcase: igt/pm_rps/reset
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468397438-21226-1-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>